Method for preparing steel sample for low-temperature tensile test, and method for low-temperature tensile test of steel bar

Abstract

The application discloses a preparation method of a steel sample for a low-temperature tensile test, and a low-temperature tensile test method of a steel bar. The preparation method comprises the following steps: taking a steel bar to be tested, and testing yield strength Y1 of the steel bar at normal temperature; selecting a clamping section steel bar, wherein the clamping section steel bar satisfies yield strength Y2 at normal temperature, Y2 >= Y1+100*b, and the coefficient b is 0.5-1.5; and cutting a test section steel bar from the steel bar to be tested, and welding the clamping section steel bar to both ends of the test section steel bar through a backing strip welding, so as to obtain the steel sample. The application greatly reduces the possibility that the steel sample is broken at the clamping section or a welding joint during the low-temperature tensile test, and improves the test success rate. Moreover, the method can be applied to the low-temperature tensile test of steel bars with different strength levels, and does not need to reform an existing low-temperature tensile testing machine, and has universal applicability.

Description

Technical Field

[0001] This application belongs to the field of steel material test sample preparation technology, specifically relating to a method for preparing steel bar specimens for low-temperature tensile testing and a method for low-temperature tensile testing of steel bars. Background Technology

[0002] In recent years, LNG storage tank projects have developed rapidly, leading to an increase in the demand for LNG storage tanks and consequently, a surge in the use of reinforcing steel. Since LNG storage tanks are primarily used in cryogenic environments, this places higher demands on the low-temperature performance of the reinforcing steel.

[0003] Low-temperature tensile testing is an important method for characterizing the low-temperature performance of reinforcing steel. The strength of reinforcing steel increases as temperature decreases. During low-temperature tensile testing, the test section is located inside the test chamber. Because the temperature inside the chamber is low, while the clamps of the tensile testing machine are located outside the chamber, holding the specimen in an environment with a higher temperature than inside the chamber, the specimen often fractures at the clamped section, rendering the test data invalid.

[0004] However, mechanical processing of reinforcing bars is not permitted during low-temperature tensile testing to avoid damaging their surface structure. Currently, this problem is mainly solved by modifying the clamps of the tensile testing machine when conducting low-temperature tensile tests on reinforcing bars. However, modifying the clamps has problems such as complex procedures and poor results, which limits the implementation of low-temperature tensile tests on reinforcing bars. Summary of the Invention

[0005] The purpose of this application is to provide a method for preparing steel bar specimens for low-temperature tensile testing and a method for conducting low-temperature tensile testing on steel bars.

[0006] To achieve one of the above-mentioned objectives, one embodiment of this application provides a method for preparing a steel bar specimen for a low-temperature tensile test, the method comprising:

[0007] Take the steel bar to be tested and test its yield strength Y1 at room temperature; Select the clamping section reinforcement, wherein the clamping section reinforcement satisfies the yield strength Y2≥Y1+100×b at room temperature, and the coefficient b=0.5~1.5; A test section of the reinforcing bar is cut from the reinforcing bar to be tested, and clamping sections of the reinforcing bar are welded to both ends of the test section by bracing, thus obtaining the reinforcing bar sample.

[0008] In one embodiment, the preparation method further includes: The tensile strength T1 of the steel bar to be tested and the tensile strength T2 of the clamped section steel bar are tested at room temperature, satisfying T2≥T1+140×c, with coefficient c=1~2.

[0009] In one embodiment, the preparation method further includes: Establish a correspondence between coefficients c and / or b and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that as the test temperature T decreases, the value of coefficient b and / or the value of coefficient c increases accordingly. Based on the target value of the test temperature T, the value of coefficient c and / or the value of coefficient b are determined using the aforementioned correspondence.

[0010] In one embodiment, the test temperature T is a Celsius value below 0°C; The correspondence includes: a first relation function c = 1 + 0.005 × |T|, and / or a second relation function b = 0.5 + 0.005 × |T|; Where |T| represents the absolute value of the test temperature T.

[0011] In one embodiment, the preparation method further includes: The strength-to-yield ratio and elongation after fracture A2 of the clamped steel bars were tested at room temperature, and the strength-to-yield ratio was ≥1.25 and the elongation after fracture A2 was ≥15%.

[0012] In one embodiment, the step of "cutting the reinforcing bar to be tested as a test section, and welding clamping sections of reinforcing bar to both ends of the test section by splice welding to obtain the reinforcing bar sample" includes: Test sections and bracing bars are cut from the steel bar to be tested. Clamping sections are then joined at both ends of the test section, and bracing bars are used as the welding support structure to perform bracing welding, thus obtaining the steel bar sample.

[0013] In one embodiment, the length of the reinforcing bar is controlled as L3 = d × D3, where d = 6~8.

[0014] In one embodiment, the length L2 of the clamping section reinforcing bar E2 is controlled to be (100+L3 / 2)mm~(150+L3 / 2)mm.

[0015] In one embodiment, the splice welding is performed using double-sided splice welding.

[0016] In one embodiment, during the bar welding, the test section reinforcement and the clamping section reinforcement are coaxially arranged, and the end face distance between the test section reinforcement and the clamping section reinforcement is 2~3mm.

[0017] In one embodiment, the welding electrode used for butt welding satisfies the requirement that the tensile strength T4 at room temperature is greater than or equal to T1, where T1 is the tensile strength of the reinforcing bar to be tested at room temperature.

[0018] In one embodiment, the current during bar welding is controlled to be 100~150A and the voltage to be 20~30V.

[0019] To achieve one of the above-mentioned objectives, one embodiment of this application provides a method for preparing a steel bar specimen for a low-temperature tensile test, the method comprising: The inner cavity length H of the test chamber used for the low-temperature tensile test is collected; The reinforcing bars to be tested are cut into test sections and reinforcement bars respectively, and the diameter D1 of the test section is collected. The length L1 of the test section and the length L3 of the reinforcement bars are controlled to satisfy L1≤H+L3-0.5×D1. The length direction of the inner cavity of the test chamber is consistent with the length direction of the test section. Remove the clamped section of reinforcing steel; The two ends of the test section steel bar are respectively connected to clamping section steel bars, and the splice steel bars are used as welding support structures to perform splice welding to obtain the steel bar sample.

[0020] In one implementation, L1≥200+6×D1+L3.

[0021] To achieve one of the above-mentioned objectives, one embodiment of this application also provides a method for low-temperature tensile testing of reinforcing bars, the method comprising the steps of: The steel bar specimen is prepared using the method described above for preparing steel bar specimens for low-temperature tensile testing. The two ends of the test section steel bar form welded joints, and the part located between the two welded joints is the effective test section. The clamps of the tensile testing machine clamp the clamping sections of the steel bar at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber. The test chamber is cooled to the test temperature T and then held at that temperature before tensile testing.

[0022] In one embodiment, the step of "cooling the test chamber to the test temperature T and then maintaining the temperature" includes: The test chamber was cooled with liquid nitrogen, and the holding time t after reaching the test temperature T was 20~40 min; Where T is -196℃ to 0℃.

[0023] In one embodiment, the test method further includes: A correspondence is established between the heat preservation time t, the diameter D1 of the steel bar to be tested, and the test temperature T of the low-temperature tensile test. The correspondence is configured such that: as the diameter D1 of the steel bar to be tested increases, the value of the heat preservation time t increases accordingly; as the test temperature T decreases, the value of the heat preservation time t increases accordingly. Based on the target values ​​of the diameter D1 of the steel bar to be tested and the test temperature T, the value of the heat preservation time t is determined using the aforementioned correspondence.

[0024] In one embodiment, the test temperature T is a Celsius value below 0°C; The correspondence includes: the third relation function t = 15 + 0.003 × D1 × |T|; Where |T| represents the absolute value of the test temperature T, and the unit of t is min.

[0025] In one embodiment, during stretching, the stretching rate before yielding is controlled as v1 = 1.8 + 0.054 × D1, in mm / min; and the stretching rate after yielding is controlled as v2 = 12 + 0.36 × D1, in mm / min.

[0026] Compared with the prior art, the beneficial effects of this application include: (1) Select the clamping section steel bar according to the strength of the steel bar to be tested at room temperature, and control the relationship between the strength of the clamping section steel bar and the test section steel bar at room temperature, and use the butt welding to make the clamping section steel bar and the welded joint have higher strength than the test section steel bar at room temperature. Even if the strength of the test section steel bar increases in a lower temperature environment, the possibility of the steel bar specimen breaking at the clamping section or the welded joint during the low temperature tensile test is greatly reduced, thus improving the success rate of the test. Moreover, this method can be applied to the low temperature tensile test of steel bars of different strength grades, and there is no need to modify the existing low temperature tensile testing machine or to machine the steel bar to be tested. It has universal applicability.

[0027] (2) Since both ends of the test section steel bar are welded joints by bracing, the part of the test section steel bar located between the welded joints is the effective test section. By limiting L1≤H+L3-0.5×D1, it can be ensured that the effective test section is entirely located in the test chamber. Furthermore, the effective test section is always entirely located in the test chamber during the tensile test, thereby ensuring the temperature uniformity of the effective test section throughout the tensile test and improving the test stability of the low temperature tensile test.

[0028] (3) When the steel bar specimen prepared by the above preparation method is subjected to low temperature tensile test, the effective test section of the test section of the steel bar is in the low temperature test chamber, while the clamping section of the steel bar is outside the test chamber with a relatively high temperature, so there is no need to modify the existing low temperature tensile testing machine. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the method for preparing a steel bar specimen for low-temperature tensile testing according to an embodiment of this application. Detailed Implementation

[0030] The technical solution of this application will be further described below with reference to specific implementation methods, but the scope of protection is not limited to the descriptions provided.

[0031] Example 1 Research has shown that the strength of steel bars increases as temperature decreases. When the temperature decreases, the thermal motion of atoms within the steel bar weakens, the atomic diffusion coefficient decreases, grain boundary migration becomes more difficult, and dislocation movement is hindered, thus increasing the strength of the steel bar. Furthermore, the lower the temperature, the more significant the increase in steel bar strength.

[0032] When conducting low-temperature tensile tests on reinforcing bars, fractures often occur at the clamping section.

[0033] In view of this, this embodiment provides a method for preparing steel bar specimens for low-temperature tensile testing to solve the problem that steel bar specimens always break at the clamping section located outside the test chamber during low-temperature tensile testing.

[0034] See Figure 1 The preparation method includes the following steps: Take the steel bar to be tested and test its yield strength Y1 at room temperature.

[0035] Select the clamping section reinforcement E2, which satisfies the yield strength Y2≥Y1+100×b at room temperature, with a coefficient b=0.5~1.5.

[0036] The test section E1 is cut from the steel bar to be tested, and the clamping sections E2 are welded to both ends of the test section E1 by bracing, thus obtaining the steel bar sample.

[0037] Understandably, the strength of the test section reinforcement E1 is the same as the strength of the reinforcement to be tested. That is to say, the yield strength of the test section reinforcement E1 at room temperature is also Y1.

[0038] Thus, by selecting the clamping section steel bar E2 based on the strength of the steel bar to be tested at room temperature, and by controlling the relationship between the strength of the clamping section steel bar E2 and the test section steel bar E1 at room temperature, and by using butt welding, the clamping section steel bar E2 and the welded joint have higher strength than the test section steel bar E1 at room temperature. Even if the strength of the test section steel bar E1 increases in a lower temperature environment, the possibility of the steel bar specimen breaking at the clamping section or the welded joint during the low-temperature tensile test is greatly reduced, thus improving the success rate of the test. Moreover, this method can be applied to the low-temperature tensile test of steel bars of different strength grades, and does not require modification of the existing low-temperature tensile testing machine or machining of the steel bar to be tested, making it universally applicable.

[0039] The yield strengths Y1 and Y2 of the reinforcing bar under test and the clamped section reinforcing bar E2 at room temperature are obtained according to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, and the unit is MPa.

[0040] The preparation method further includes the following steps: The surface of the test section steel bar E1 was cut.

[0041] Specifically, according to GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks, the surface of the test section steel bar E1 was notched.

[0042] In one embodiment, the preparation method further includes: The tensile strength T1 of the steel bar to be tested and the tensile strength T2 of the clamped section steel bar E2 are tested at room temperature, satisfying T2≥T1+140×c, with coefficient c=1~2.

[0043] It is understandable that the tensile strength of the test section steel bar E1 is the same as that of the steel bar to be tested, both being T1.

[0044] Thus, by controlling the relationship between the tensile strength of the clamping section steel bar E2 and the steel bar to be tested at room temperature, the clamping section steel bar E2 is selected so that the clamping section steel bar E2 has a higher tensile strength at room temperature than the test section steel bar E1. Even if the tensile strength of the test section steel bar E1 increases in a lower temperature environment, the possibility of the steel bar specimen breaking at the clamping section during the low-temperature tensile test is greatly reduced, thus improving the success rate of the test. Moreover, it can be applied to the low-temperature tensile test of steel bars of different strength grades and has universal applicability.

[0045] The tensile strengths T1 and T2 of the reinforcing bar under test and the clamped section reinforcing bar E2 at room temperature are obtained according to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, and the unit is MPa.

[0046] The preparation method further includes: Establish a correspondence between coefficients c and / or b and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that as the test temperature T decreases, the value of coefficient b and / or the value of coefficient c increases accordingly. Based on the target value of the test temperature T, the value of coefficient c and / or the value of coefficient b are determined using the aforementioned correspondence.

[0047] Thus, by establishing the correspondence between coefficients c and b and the test temperature T of the low-temperature tensile test, the strength range that the clamped section steel bar E2 needs to meet can be determined based on the test temperature T of the low-temperature tensile test. This allows for the selection of a suitable clamped section steel bar E2 for different test temperatures, further reducing the possibility of fracture occurring at the clamped section steel bar E2.

[0048] Wherein, the test temperature T is a Celsius value below 0°C; The correspondence includes: a first relation function c = 1 + 0.005 × |T|, and / or a second relation function b = 0.5 + 0.005 × |T|; Where |T| represents the absolute value of the test temperature T.

[0049] For example, if the test temperature T is -100℃, then |T| takes the value 100.

[0050] By defining the functional relationship between the limiting coefficients b and c and the test temperature T of the low-temperature tensile test, a basis is provided for selecting the appropriate clamping section reinforcement E2.

[0051] In one embodiment, the preparation method further includes: A correspondence is established between the coefficient b and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that the value of the coefficient b increases as the test temperature T decreases. Based on the target value of the test temperature T, the value of coefficient b is determined using the aforementioned correspondence.

[0052] Specifically, the correspondence includes: the second relation function b = 0.5 + 0.005 × |T|.

[0053] In another embodiment, the preparation method further includes: A correspondence is established between the coefficient c and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that the value of the coefficient c increases as the test temperature T decreases. Based on the target value of the test temperature T, the value of coefficient c is determined using the aforementioned correspondence.

[0054] Specifically, the correspondence includes: a first relational function c = 1 + 0.005 × |T|.

[0055] In yet another embodiment, the preparation method further includes: A correspondence is established between coefficients c and b and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that as the test temperature T decreases, the value of coefficient b increases accordingly, and the value of coefficient c also increases accordingly. Based on the target value of the test temperature T, the values ​​of coefficient b and coefficient c are determined using the aforementioned correspondence.

[0056] Specifically, the correspondence includes: a first relation function c = 1 + 0.005 × |T|, and a second relation function b = 0.5 + 0.005 × |T|.

[0057] In one embodiment, the preparation method further includes: The strength-to-yield ratio and elongation after fracture of the clamped section steel bar E2 were tested at room temperature, and the strength-to-yield ratio of the clamped section steel bar E2 at room temperature was ≥1.25 and the elongation after fracture was ≥15%.

[0058] By selecting the clamping section steel bar E2 to meet the requirements of a strength-to-yield ratio ≥1.25 at room temperature and an elongation after fracture A2 ≥15%, we can further provide data basis for the selection of various performance indicators of the clamping section steel bar E2, and further reduce the possibility that the fracture location will occur at the clamping section steel bar E2 during the low temperature tensile test.

[0059] Among them, the strength-to-yield ratio and elongation after fracture of the clamped section steel bar E2 at room temperature are obtained according to GB / T228.1-2021 Metallic materials, tensile testing method, part 1: room temperature test method.

[0060] In one embodiment, the step of "cutting the reinforcing bar to be tested as the test section reinforcing bar E1, and welding clamping sections E2 to both ends of the test section reinforcing bar E1 by splice welding to obtain the reinforcing bar sample" includes: Test section E1 and splice bar E3 are cut from the steel bar to be tested. The clamping section E2 is then joined at both ends of the test section E1. The splice bar E3 is used as the welding support structure for splice welding to obtain the steel bar sample.

[0061] In this way, the splice bar E3 and the test section bar E1 come from the same bar, and their materials and diameters are the same. That is, the diameter D1 of the test section bar E1 and the diameter D3 of the splice bar E3 are the same as the diameter of the bar to be tested. This eliminates the need to select a suitable splice bar separately, thereby improving efficiency.

[0062] By using the test section steel bar E1 and the clamping section steel bar E2 as the main reinforcement, the splice steel bar E3 is attached to the outside of the joint of E1 and E2. Welding is performed at the edge where the splice steel bar contacts the main reinforcement to form a weld, so that the splice steel bar and the main reinforcement are fused into a whole. That is, the splice steel bar E3, the test section steel bar E1, and the clamping section steel bar E2 are fused together to form a welded joint.

[0063] The diameter of the steel bar to be tested is 16 - 40 mm to meet the requirements of the steel bar low-temperature tensile test for the specimen. Then, the diameter D1 of the test section steel bar E1 and the diameter D3 of the tie bar steel bar E3 are both 16 - 40 mm.

[0064] The diameter D2 of the clamping section steel bar E2 = the diameter D1 of the test section steel bar E1. This can make the tie bar steel bar E3 fit the test section steel bar E1 and the clamping section steel bar E2 simultaneously, improving the welding strength.

[0065] The diameter of the steel bar to be tested, the diameter D1 of the test section steel bar E1, the diameter D2 of the clamping section steel bar E2, and the diameter D3 of the tie bar steel bar E3 are all nominal diameters.

[0066] In one embodiment, control the side fillet welding process so that the welded joint formed by the fusion of the tie bar steel bar E3, the test section steel bar E1, and the clamping section steel bar E2 satisfies: the tensile strength T0 at room temperature ≥ 161 + 1.15×T1 + 0.8×|T|.

[0067] In this way, the strength of the welded joint formed by the fusion of the tie bar steel bar E3, the test section steel bar E1, and the clamping section steel bar E2 at room temperature is greater than the strength of the test section steel bar E1 at room temperature, thereby reducing the possibility of fracture occurring at the welded joint during the low-temperature tensile test.

[0068] Among them, the tensile strength T0 of the welded joint at room temperature is obtained by testing according to GB / T228.1 - 2021 Metallic materials - Tensile testing - Part 1: Method of test at room temperature, and the unit is MPa.

[0069] In one embodiment, control the side fillet welding process so that the welded joint formed by the fusion of the tie bar steel bar E3, the test section steel bar E1, and the clamping section steel bar E2 satisfies: the V-notch impact energy value at the test temperature T of the steel bar low-temperature tensile test ≥ 100 J, so as to reduce the possibility of the steel bar specimen breaking at the welded joint during the low-temperature tensile test.

[0070] Among them, the V-notch impact energy value is tested according to GB / T 29 - 2007 Metallic materials - Charpy pendulum impact test method.

[0071] The length L3 of the tie bar steel bar E3 < the length L1 of E1. In this way, it is possible to avoid the length of the tie bar steel bar E3 being too long and affecting the low-temperature tensile test.

[0072] In one embodiment, control the length L3 of the tie bar steel bar E3 = d×D3, where d = 6 - 8, and the units of D3 and L3 are both mm.

[0073] By controlling the ratio between the length L3 of the reinforcing bar E3 and its diameter D3, the welded joint can be ensured to be strong. If the length of the reinforcing bar E3 is too small, the welded joint may not be strong enough and may crack easily during tensile testing. If the length of the reinforcing bar E3 is too large, the effective test section between the two welded joints may not be long enough, which will affect the normal conduct of the low-temperature tensile test.

[0074] In one embodiment, the length L2 of the clamping section of the reinforcing bar E2 is controlled to be (100+L3 / 2)mm to (150+L3 / 2)mm. This ensures that the clamping section of the reinforcing bar E2 still has sufficient length for the tensile testing machine's clamps after the splice is welded.

[0075] During siding welding, the test section reinforcing bar E1 and the clamping section reinforcing bar E2 are arranged coaxially, and the end face distance between the test section reinforcing bar E1 and the clamping section reinforcing bar E2 is controlled to be 2~3mm. A siding reinforcing bar E3 is added to the side of the joint as a welding support structure before siding welding. This can prevent misalignment of the joint end faces of E1 and E2 after welding and ensure the connection strength of the welded joint.

[0076] Before welding the reinforcing bars, the end faces of the test section E1 and the clamping section E2 need to be mechanically polished to ensure that the end faces are smooth and flat, so as to improve the connection strength of the welded joint.

[0077] In one embodiment, double-sided splicing is used for the splice welding. That is, splice bars E3 are added on both radial sides of the joint between the test section steel bar E1 and the clamping section steel bar E2 as welding support structures, and then welding is performed. In this way, by controlling the splice welding process, the connection strength of the welded joint can be improved, and the circumferential strength uniformity of the welded joint can also be improved.

[0078] The welding rods used for butt welding must have a tensile strength T4 at room temperature that is greater than or equal to the tensile strength T1 of the test section of reinforcing steel E1 at room temperature. Thus, by controlling the butt welding process, the strength of the welded joint at room temperature is ensured to be no less than the strength of the test section of reinforcing steel E1, reducing the possibility of fracture at the welded joint during low-temperature tensile tests.

[0079] Specifically, the welding rod needs to be selected based on the chemical composition design scheme and carbon equivalent of the test section steel bar E1, the clamping section steel bar E2, and the bracing bar steel bar E3.

[0080] In one embodiment, the current during bar welding is controlled to be 100-150A and the voltage to be 20-30V. By controlling the current and voltage during the bar welding process, efficient melting of the welding rod and the reinforcing bar can be ensured. If the current and voltage during welding are too low, the energy efficiency will be insufficient, and the welding rod and the reinforcing bar will not be able to melt; if the current and voltage during welding are too high, burn-through and spatter are likely to occur, affecting the quality of the weld joint and resulting in insufficient weld joint strength.

[0081] Example 2 This embodiment also provides a method for preparing steel bar specimens for low-temperature tensile testing, the method comprising: The inner cavity length H of the test chamber used for the low-temperature tensile test is collected; The steel bars to be tested are cut into test sections E1 and E3 respectively, and the diameter D1 of the test section E1 is collected. The length L1 of the test section E1 and the length L3 of the E3 are controlled to satisfy L1≤H+L3-0.5×D1. The length direction of the inner cavity of the test chamber is consistent with the length direction of the test section E1. Take the clamping section of steel bar E2; The two ends of the test section steel bar E1 are respectively connected to the clamping section steel bar E2, and the splice bar steel bar E3 is used as the welding support structure to perform splice welding to obtain the steel bar sample.

[0082] Since both ends of the test section steel bar E1 form welded joints along its length, the portion of the test section steel bar E1 located between the welded joints at both ends is the effective test section. By limiting L1≤H+L3-0.5×D1, it can be ensured that the entire effective test section is located inside the test chamber, and the entire effective test section is always located inside the test chamber during the tensile test. This ensures the temperature uniformity of the effective test section throughout the tensile test and improves the test stability.

[0083] Since the reinforcement bar E3 and the test section reinforcement bar E1 come from the same reinforcement bar, their materials and diameters are the same. That is, the diameter D1 of the test section reinforcement bar E1 and the diameter D3 of the reinforcement bar E3 are the same as the diameter of the reinforcement bar to be tested.

[0084] During the low-temperature tensile test of steel bars, the maximum total elongation at force (Agt) of the steel bar under test can be measured manually. However, it is necessary to measure the two sections of the steel bar after fracture separately, and the measurement positions must be selected at a certain distance from the fracture and from the clamping end.

[0085] In one embodiment, the length L1 of the test section of the reinforcing bar E1 is controlled to be ≥200+6×D1+L3. This ensures that the test section of the reinforcing bar E1 has a sufficiently long effective test section, so that after fracture, the test section of the reinforcing bar still has enough length for manual measurement of the maximum force total elongation Ag of the reinforcing bar under test, thereby avoiding the need for additional cryogenic extensometers and saving costs.

[0086] The above content is the difference between this embodiment 2 and embodiment 1.

[0087] Except for the differences mentioned above, all the contents of Example 1 are applicable to this Example 2, and will not be repeated here.

[0088] Example 3 This embodiment provides a low-temperature tensile test method for reinforcing bars, which is carried out in accordance with GB / T 228.3-2019 Tensile Testing of Metallic Materials.

[0089] Specifically, the low-temperature tensile test method for the reinforcing steel bars includes the following steps: S21. Sample preparation The steel bar specimen was prepared using the method described in Example 1 or 2 above for preparing steel bar specimens for low-temperature tensile testing. Welded joints were formed at both ends of the test section steel bar E1, and the portion of the steel bar specimen located between the two welded joints constituted the effective test section.

[0090] S22. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0091] S23. Cool the test chamber to the test temperature T, hold it at that temperature, and then perform the tensile test.

[0092] Thus, when the specimen prepared by the aforementioned method is subjected to a low-temperature tensile test, the effective test section of the test section E1 of the steel bar is located inside the low-temperature test chamber, while the clamping section E2 of the steel bar is located outside the test chamber at a relatively high temperature, thereby eliminating the need to modify the existing low-temperature tensile testing machine.

[0093] The tensile properties of the reinforcing steel at low temperatures can be determined using a low-temperature tensile test method. These properties include, but are not limited to, yield strength (Y) and tensile strength (T).

[0094] Furthermore, the low-temperature tensile test method for the reinforcing steel also includes the following steps: S24. After the steel bar specimen has been removed from the tensile fracture test and the temperature has returned to room temperature, the maximum total elongation at force Agt and the elongation at fracture A of the steel bar specimen shall be measured manually in accordance with GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1 - Room Temperature Test Method.

[0095] Preferably, the step of "cooling the test chamber to the test temperature T and then maintaining the temperature" includes: The test chamber was cooled with liquid nitrogen, and the holding time t after reaching the test temperature T was 20~40 min, where T was -196℃~0℃.

[0096] In other words, the applicable temperature range for this low-temperature tensile test method is -196℃ to 0℃, meaning that this application can be used to test the low-temperature tensile properties of steel bars within the temperature range of -196℃ to 0℃.

[0097] Cooling the test chamber with liquid nitrogen allows for temperature control within the chamber according to testing requirements, with a relatively low minimum achievable temperature. Furthermore, controlling the holding time improves the consistency between the core and surface temperatures of the steel reinforcement sample, thereby enhancing the accuracy of the test results.

[0098] Specifically, liquid nitrogen is used to cool the test chamber from the outside. This improves the temperature uniformity inside the test chamber, thereby improving the temperature uniformity of the test section of steel bar E1 along its length.

[0099] In step S23, a temperature testing device is installed inside the test chamber, and the distance between the temperature testing device and the surface of the test section steel bar E1 is controlled to be 2-3 mm. This allows for real-time monitoring of the temperature inside the test chamber, especially the temperature around the test section steel bar E1, thereby improving the accuracy of temperature control.

[0100] In one embodiment, the low-temperature tensile test method for the reinforcing steel bar further includes: A correspondence is established between the heat preservation time t, the diameter D1 of the steel bar to be tested, and the test temperature T of the low-temperature tensile test. The correspondence is configured such that: as the diameter D1 of the steel bar to be tested increases, the value of the heat preservation time t increases accordingly; as the test temperature T decreases, the value of the heat preservation time t increases accordingly. Based on the target values ​​of the diameter D1 of the steel bar to be tested and the test temperature T, the value of the heat preservation time t is determined using the aforementioned correspondence.

[0101] In this way, the heat preservation time can be controlled according to the diameter D1 of the test section steel bar E1 (that is, the diameter D1 of the steel bar to be tested), so as to ensure that the core temperature of the steel bar sample is consistent with the surface temperature and improve the accuracy of the test results.

[0102] In one embodiment, the test temperature T is a Celsius value below 0°C; The correspondence includes: the third relation function t = 15 + 0.003 × D1 × |T|, with the unit being min.

[0103] By defining the functional relationship between the heat preservation time t, the diameter D1 of the steel bar to be tested, and the test temperature T of the low-temperature tensile test, a basis is provided for the reasonable control of the heat preservation time.

[0104] In one embodiment, during stretching, the stretching rate before yielding is controlled as v1 = 1.8 + 0.054 × D1, in mm / min; and the stretching rate after yielding is controlled as v2 = 12 + 0.36 × D1, in mm / min.

[0105] By defining the functional relationship between the tensile rate before and after yielding and the diameter D1 of the steel bar to be tested, a reasonable tensile rate can be set according to different diameters of the steel bars to be tested, so as to improve the test accuracy and test efficiency.

[0106] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application, and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

[0107] The following specific test examples and comparative examples further illustrate the beneficial effects of this application. Of course, these test examples are only a part of the many variations contained in this application, and not all of them.

[0108] Experimental Example 1 This experimental example follows the same method for preparing steel bar specimens for low-temperature tensile testing as described above, specifically including the following steps: S11a. Take the steel bar to be tested, and cut out the test section steel bar E1 and the splice steel bar E3 from the steel bar to be tested.

[0109] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 and tensile strength T1 of the tested steel bar at room temperature were measured to be 454 MPa and 577 MPa, respectively.

[0110] The diameter of the reinforcing bar to be tested was measured to be 20mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 20mm.

[0111] The inner cavity length H of the test chamber used for the low-temperature tensile test was measured to be 400 mm.

[0112] The length of the control bar reinforcement E3 is L3=7×D3=140mm.

[0113] The length L1 of the test section reinforcement E1 must meet the following requirements: 460mm≤L1≤530mm.

[0114] In this test example, the length L1 of the control test section reinforcement E1 is 500mm.

[0115] S12a. Select the clamping section reinforcement E2.

[0116] The target value for the test temperature T in this low-temperature tensile test is -100℃. That is, the low-temperature tensile test in this example is conducted at -100℃.

[0117] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength, tensile strength, strength-to-yield ratio, and elongation after fracture of different reinforcing bars at room temperature were tested.

[0118] The steel bar with a yield strength ≥554MPa, tensile strength ≥787MPa, strength-to-yield ratio ≥1.25, elongation after fracture ≥15%, and the same diameter as D1 was selected as the clamping section steel bar E2.

[0119] The length L2 of the clamping section of the reinforcing bar E2 must be within the range of 170mm to 220mm.

[0120] In this test example, the diameter D2 of the clamping section steel bar E2 is 20mm, the length L2 is 190mm, the yield strength Y2 at room temperature is 568MPa, the tensile strength T2 is 798MPa, the strength-to-yield ratio is 1.40, and the elongation after fracture A2 is 18%.

[0121] S13a. The surface of the test section steel bar E1 is notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0122] S14a. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0123] The test section steel bar E1 is connected to the clamping section steel bar E2 at both ends. The test section steel bar E1 and the clamping section steel bar E2 are used as the main reinforcement. The test section steel bar E1 and the clamping section steel bar E2 are arranged coaxially, and the end face distance between the test section steel bar E1 and the clamping section steel bar E2 is controlled to be 2~3mm. The splice steel bar E3 is used as the welding support structure. Double-sided splice welding is performed at the joint of the test section steel bar E1 and the clamping section steel bar E2 to obtain the steel bar sample.

[0124] Select low-alloy steel welding electrodes with a tensile strength ≥ 577 MPa at room temperature.

[0125] Welding was performed using different welding currents and voltages, without post-weld heat treatment. The tensile strength T0 of the welded joint at room temperature was tested according to GB / T228.1-2021 Metallic materials, tensile testing—Part 1: Room temperature test method. The V-notch impact energy of the welded joint at the test temperature of the low-temperature tensile test of reinforcing steel was tested according to GB / T 29-2007 Metallic materials, Charpy pendulum impact test method.

[0126] Select welding current and welding voltage parameters that meet the requirements of a tensile strength T0 ≥ 910 MPa at room temperature and a V-notch impact energy ≥ 100 J at the test temperature of the low-temperature tensile test of the reinforcing steel.

[0127] In this experimental example, a low-alloy steel welding electrode with a tensile strength (T4) of 590 MPa at room temperature was selected, and the welding current was controlled at 140 A and the voltage at 25 V. No heat treatment was required after welding.

[0128] Furthermore, the low-temperature tensile testing method for reinforcing bars described above was used to conduct low-temperature tensile tests on the aforementioned specimens. The specific steps are as follows: S21a. Sample preparation The steel bar specimens were prepared according to the aforementioned method for preparing steel bar specimens for low-temperature tensile testing. Welded joints were formed at both ends of the test section steel bar E1, and the portion of the steel bar specimen located between the two welded joints constituted the effective test section.

[0129] S22a. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0130] S23a. The test chamber is cooled from the outside by liquid nitrogen to -100℃ and then kept at that temperature for 21 minutes.

[0131] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once all three devices have measured temperatures of -100℃±1℃.

[0132] S24a. Set the tensile rate v1 before yielding to 2.88 mm / min and the tensile rate v2 after yielding to 19.2 mm / min. Perform tensile testing.

[0133] The fracture occurred within the effective test section of the reinforcing bar specimen. The yield strength Y of the tested section of reinforcing bar E1 at -100℃ was measured to be 542 MPa, and the tensile strength T was 722 MPa.

[0134] S25a. Open the test chamber, take out the fractured specimen, and wait for the temperature to return to room temperature. According to GB / T228.1-2021 Metallic materials, tensile testing - Part 1: Room temperature test method, use manual measurement to measure the maximum force total elongation Agt of the specimen as 7.5% and the elongation after fracture A as 10%.

[0135] Experimental Example 2 This experimental example follows the same method for preparing steel bar specimens for low-temperature tensile testing as described above, specifically including the following steps: S11a. Take the steel bar to be tested, and cut out the test section steel bar E1 and the splice steel bar E3 from the steel bar to be tested.

[0136] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 of the tested steel bar at room temperature was 521 MPa and the tensile strength T1 was 667 MPa.

[0137] The diameter of the reinforcing bar under test was measured to be 16mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 16mm.

[0138] The inner cavity length H of the test chamber used for the low-temperature tensile test was measured to be 400 mm.

[0139] The length of the control bar reinforcement E3 is L3=8×D3=128mm.

[0140] The length L1 of the test section reinforcement E1 must meet the following requirements: 424mm≤L1≤520mm.

[0141] In this test example, the length L1 of the control test section reinforcement E1 is 500mm.

[0142] S12a. Select the clamping section reinforcement E2.

[0143] The target value for the test temperature T in this low-temperature tensile test is 0℃. That is, the low-temperature tensile test in this example is conducted at 0℃.

[0144] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength, tensile strength, strength-to-yield ratio, and elongation after fracture of different reinforcing bars at room temperature were tested.

[0145] The steel bar with a yield strength ≥ 571 MPa, tensile strength ≥ 807 MPa, strength-to-yield ratio ≥ 1.25, elongation after fracture ≥ 15%, and the same diameter as D1 was selected as the clamping section steel bar E2.

[0146] The length L2 of the clamping section of the reinforcing bar E2 must be within the range of 164mm to 214mm.

[0147] In this test example, the diameter D2 of the clamping section steel bar E2 is 16mm, the length L2 is 190mm, the yield strength Y2 at room temperature is 585MPa, the tensile strength T2 is 810MPa, the strength-to-yield ratio is 1.38, and the elongation after fracture A2 is 20%.

[0148] S13a. The surface of the test section steel bar E1 is notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0149] S14a. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0150] The test section steel bar E1 is connected to the clamping section steel bar E2 at both ends. The test section steel bar E1 and the clamping section steel bar E2 are used as the main reinforcement. The test section steel bar E1 and the clamping section steel bar E2 are arranged coaxially, and the end face distance between the test section steel bar E1 and the clamping section steel bar E2 is controlled to be 2~3mm. The splice steel bar E3 is used as the welding support structure. Double-sided splice welding is performed at the joint of the test section steel bar E1 and the clamping section steel bar E2 to obtain the steel bar sample.

[0151] Select low-alloy steel welding electrodes with a tensile strength ≥667MPa at room temperature.

[0152] Welding was performed using different welding currents and voltages, without post-weld heat treatment. The tensile strength T0 of the welded joint at room temperature was tested according to GB / T228.1-2021 Metallic materials, tensile testing—Part 1: Room temperature test method. The V-notch impact energy of the welded joint at the test temperature of the low-temperature tensile test of reinforcing steel was tested according to GB / T 29-2007 Metallic materials, Charpy pendulum impact test method.

[0153] Select welding current and welding voltage parameters that meet the requirements of a tensile strength T0 ≥ 928 MPa at room temperature and a V-notch impact energy ≥ 100 J at the test temperature of the low-temperature tensile test of the reinforcing steel.

[0154] In this experimental example, a low-alloy steel welding electrode with a tensile strength (T4) of 698 MPa at room temperature was selected, and the welding current was controlled at 150 A and the voltage at 30 V. No heat treatment was required after welding.

[0155] Furthermore, the low-temperature tensile testing method for reinforcing bars described above was used to conduct low-temperature tensile tests on the aforementioned specimens. The specific steps are as follows: S21a. Sample preparation The steel bar specimens were prepared according to the aforementioned method for preparing steel bar specimens for low-temperature tensile testing. Welded joints were formed at both ends of the test section steel bar E1, and the portion of the steel bar specimen located between the two welded joints constituted the effective test section.

[0156] S22a. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0157] S23a. The test chamber is cooled from the outside using liquid nitrogen until it reaches 0°C, and then kept at that temperature for 15 minutes.

[0158] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once the temperatures measured by all three devices reach 0℃±1℃.

[0159] S24a. Set the tensile rate v1 before yielding to 2.664 mm / min and the tensile rate v2 after yielding to 17.76 mm / min. Perform tensile testing.

[0160] The fracture occurred within the effective test section of the reinforcing bar specimen. The yield strength Y of the tested section of reinforcing bar E1 at 0℃ was measured to be 589 MPa, and the tensile strength T was 777 MPa.

[0161] S25a. Open the test chamber, take out the fractured specimen, and wait for the temperature to return to room temperature. According to GB / T228.1-2021 Metallic materials, tensile testing - Part 1: Room temperature test method, use manual measurement to measure the maximum force total elongation Agt of the specimen as 9% and the elongation after fracture as A as 15%.

[0162] Experimental Example 3 This experimental example follows the same method for preparing steel bar specimens for low-temperature tensile testing as described above, specifically including the following steps: S11a. Take the steel bar to be tested, and cut out the test section steel bar E1 and the splice steel bar E3 from the steel bar to be tested.

[0163] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 and tensile strength T1 of the tested steel bar at room temperature were measured to be 621 MPa and 801 MPa, respectively.

[0164] The diameter of the reinforcing bar to be tested was measured to be 25mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 25mm.

[0165] The inner cavity length H of the test chamber used for the low-temperature tensile test was measured to be 400 mm.

[0166] The length of the control bar reinforcement E3 is L3=6×D3=150mm.

[0167] The length L1 of the test section reinforcement E1 must meet the following requirement: 500mm≤L1≤537.5mm.

[0168] In this test example, the length L1 of the control test section reinforcement E1 is 500mm.

[0169] S12a. Select the clamping section reinforcement E2.

[0170] The target value for the test temperature T in this low-temperature tensile test is -196℃. That is, the low-temperature tensile test in this example is conducted at -196℃.

[0171] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength, tensile strength, strength-to-yield ratio, and elongation after fracture of different reinforcing bars at room temperature were tested.

[0172] The steel bar with a yield strength ≥769MPa, tensile strength ≥1078.2MPa, strength-to-yield ratio ≥1.25, elongation after fracture ≥15%, and the same diameter as D1 was selected as the clamping section steel bar E2.

[0173] The length L2 of the clamping section of the reinforcing bar E2 must be within the range of 175mm to 225mm.

[0174] In this test example, the diameter D2 of the clamping section steel bar E2 is 25mm, the length L2 is 190mm, the yield strength Y2 at room temperature is 790MPa, the tensile strength T2 is 1100MPa, the strength-to-yield ratio is 1.39, and the elongation after fracture A2 is 19%.

[0175] S13a. The surface of the test section steel bar E1 is notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0176] S14a. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0177] The test section steel bar E1 is connected to the clamping section steel bar E2 at both ends. The test section steel bar E1 and the clamping section steel bar E2 are used as the main reinforcement. The test section steel bar E1 and the clamping section steel bar E2 are arranged coaxially, and the end face distance between the test section steel bar E1 and the clamping section steel bar E2 is controlled to be 2~3mm. The splice steel bar E3 is used as the welding support structure. Double-sided splice welding is performed at the joint of the test section steel bar E1 and the clamping section steel bar E2 to obtain the steel bar sample.

[0178] Select low-alloy steel welding electrodes with a tensile strength ≥ 801 MPa at room temperature.

[0179] Welding was performed using different welding currents and voltages, without post-weld heat treatment. The tensile strength T0 of the welded joint at room temperature was tested according to GB / T228.1-2021 Metallic materials, tensile testing—Part 1: Room temperature test method. The V-notch impact energy of the welded joint at the test temperature of the low-temperature tensile test of reinforcing steel was tested according to GB / T 29-2007 Metallic materials, Charpy pendulum impact test method.

[0180] Select welding parameters such as welding current and welding voltage that meet the requirements of a tensile strength T0 ≥ 1238.95 MPa at room temperature and a V-notch impact energy ≥ 100 J at the test temperature of the low-temperature tensile test of steel bars.

[0181] In this experimental example, a low-alloy steel welding electrode with a tensile strength (T4) of 835 MPa at room temperature was selected, and the welding current was controlled at 100 A and the voltage at 20 V. No heat treatment was required after welding.

[0182] Furthermore, the low-temperature tensile testing method for reinforcing bars described above was used to conduct low-temperature tensile tests on the aforementioned specimens. The specific steps are as follows: S21a. Sample preparation The steel bar specimens were prepared according to the aforementioned method for preparing steel bar specimens for low-temperature tensile testing. Welded joints were formed at both ends of the test section steel bar E1, and the portion of the steel bar specimen located between the two welded joints constituted the effective test section.

[0183] S22a. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0184] S23a. The test chamber is cooled by liquid nitrogen from the outside to -196℃ and then kept at that temperature for 29.7 min.

[0185] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once all three devices have measured a temperature of -196℃±1℃.

[0186] S24a. Set the tensile rate v1 before yielding to 3.15 mm / min and the tensile rate v2 after yielding to 21 mm / min. Perform tensile testing.

[0187] The fracture occurred within the effective test section of the reinforcing bar specimen. The yield strength Y of the tested section of reinforcing bar E1 at -196℃ was measured to be 768 MPa, and the tensile strength T was 1035 MPa.

[0188] S25a. Open the test chamber, take out the fractured specimen, and wait for the temperature to return to room temperature. According to GB / T228.1-2021 Metallic materials, tensile testing - Part 1: Room temperature test method, use manual measurement to measure the maximum force total elongation Agt of the specimen as 5% and the elongation after fracture A as 8%.

[0189] Test Example 4 This experimental example follows the same method for preparing steel bar specimens for low-temperature tensile testing as described above, specifically including the following steps: S11a. Take the steel bar to be tested, and cut out the test section steel bar E1 and the splice steel bar E3 from the steel bar to be tested.

[0190] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 of the tested steel bar at room temperature was measured to be 430 MPa and the tensile strength T1 was 567 MPa.

[0191] The diameter of the reinforcing bar to be tested was measured to be 40 mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 40 mm.

[0192] The inner cavity length H of the test chamber used for the low-temperature tensile test was measured to be 500 mm.

[0193] The length of the control bar reinforcement E3 is L3=6×D3=240mm.

[0194] The length L1 of the test section reinforcement E1 must meet the following requirements: 680mm≤L1≤720mm.

[0195] In this test example, the length L1 of the control test section reinforcement E1 is 700mm.

[0196] S12a. Select the clamping section reinforcement E2.

[0197] The target value for the test temperature T in this low-temperature tensile test is -175℃. That is, the low-temperature tensile test in this example is conducted at -175℃.

[0198] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength, tensile strength, strength-to-yield ratio, and elongation after fracture of different reinforcing bars at room temperature were tested.

[0199] The steel bar with a yield strength ≥ 567.5 MPa, tensile strength ≥ 829.5 MPa, strength-to-yield ratio ≥ 1.25, elongation after fracture ≥ 15%, and the same diameter as D1 was selected as the clamping section steel bar E2.

[0200] The length L2 of the clamping section of the reinforcing bar E2 must be within the range of 220mm to 270mm.

[0201] In this test example, the diameter D2 of the clamping section steel bar E2 is 40mm, the length L2 is 250mm, the yield strength Y2 at room temperature is 600MPa, the tensile strength T2 is 850MPa, the strength-to-yield ratio is 1.42, and the elongation after fracture A2 is 22%.

[0202] S13a. The surface of the test section steel bar E1 is notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0203] S14a. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0204] The test section steel bar E1 is connected to the clamping section steel bar E2 at both ends. The test section steel bar E1 and the clamping section steel bar E2 are used as the main reinforcement. The test section steel bar E1 and the clamping section steel bar E2 are arranged coaxially, and the end face distance between the test section steel bar E1 and the clamping section steel bar E2 is controlled to be 2~3mm. The splice steel bar E3 is used as the welding support structure. Double-sided splice welding is performed at the joint of the test section steel bar E1 and the clamping section steel bar E2 to obtain the steel bar sample.

[0205] Select low-alloy steel welding electrodes with a tensile strength ≥ 567 MPa at room temperature.

[0206] Welding was performed using different welding currents and voltages, without post-weld heat treatment. The tensile strength T0 of the welded joint at room temperature was tested according to GB / T228.1-2021 Metallic materials, tensile testing—Part 1: Room temperature test method. The V-notch impact energy of the welded joint at the test temperature of the low-temperature tensile test of reinforcing steel was tested according to GB / T 29-2007 Metallic materials, Charpy pendulum impact test method.

[0207] Select welding current and welding voltage parameters that meet the requirements of a tensile strength T0 ≥ 953 MPa at room temperature and a V-notch impact energy ≥ 100 J at the test temperature of the low-temperature tensile test of the reinforcing steel.

[0208] In this experimental example, a low-alloy steel welding electrode with a tensile strength (T4) of 580 MPa at room temperature was selected, and the welding current was controlled at 110 A and the voltage at 26 V. No heat treatment was required after welding.

[0209] Furthermore, the low-temperature tensile testing method for reinforcing bars described above was used to conduct low-temperature tensile tests on the aforementioned specimens. The specific steps are as follows: S21a. Sample preparation The steel bar specimens were prepared according to the aforementioned method for preparing steel bar specimens for low-temperature tensile testing. Welded joints were formed at both ends of the test section steel bar E1, and the portion of the steel bar specimen located between the two welded joints constituted the effective test section.

[0210] S22a. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0211] S23a. The test chamber is cooled from the outside using liquid nitrogen to -175℃ and then kept at that temperature for 36 minutes.

[0212] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once all three devices have measured a temperature of -175℃±1℃.

[0213] S24a. Set the tensile rate v1 before yielding to 3.96 mm / min and the tensile rate v2 after yielding to 26.4 mm / min. Perform tensile testing.

[0214] The fracture occurred within the effective test section of the reinforcing bar specimen. The yield strength Y of the tested section of reinforcing bar E1 at -175℃ was measured to be 586 MPa, and the tensile strength T was 830 MPa.

[0215] S25a. Open the test chamber, take out the fractured specimen, and wait for the temperature to return to room temperature. According to GB / T228.1-2021 Metallic materials, tensile testing - Part 1: Room temperature test method, the specimen is measured manually to obtain the maximum force total elongation Agt of 4.5% and the elongation after fracture A of 6%.

[0216] Comparative Example 1 This comparative example directly subjected the steel bar specimens to low-temperature tensile testing, and the specific steps are as follows: A section of the reinforcing bar to be tested was taken as test section E1. The diameter of the reinforcing bar was measured to be 20 mm, and its yield strength Y1 at room temperature was 454 MPa, and its tensile strength T1 was 577 MPa. Therefore, the diameter D1 of test section E1 is 20 mm, and its yield strength Y1 at room temperature is 454 MPa, and its tensile strength T1 is 577 MPa. The length L1 of the test section E1 is 690 mm.

[0217] According to GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks, the surface of the test section steel bar E1 was notched.

[0218] The clamps of the tensile testing machine hold the two ends of the test section steel bar E1 respectively, and the middle part of the test section steel bar E1 is located in the test chamber used for low temperature tensile testing.

[0219] The test chamber was cooled externally with liquid nitrogen to -100℃, and then held at that temperature for 25 minutes. Three temperature testing devices were installed inside the chamber, evenly spaced along the length of the test section of reinforcing steel E1, with each device 2mm away from the surface of the steel section. The holding time began once the temperatures measured by all three devices reached -100℃ ± 1℃.

[0220] The tensile rate v1 before yielding was set to 2.88 mm / min, and the tensile rate v2 after yielding was set to 19.2 mm / min, and tensile testing was performed.

[0221] During the tensile test, the specimen fractured from the portion located outside the test chamber. This low-temperature tensile test is invalid.

[0222] Comparative Example 2 The method for preparing the steel reinforcement specimens in this comparative example includes the following steps: S11b. Take the reinforcing bar to be tested, and cut out a test section E1 with a length L1 of 500mm and a bracing bar E3 with a length L3 of 140mm from the reinforcing bar to be tested.

[0223] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 and tensile strength T1 of the tested steel bar at room temperature were measured to be 454 MPa and 577 MPa, respectively.

[0224] The diameter of the reinforcing bar to be tested was measured to be 20mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 20mm.

[0225] S12b. Select the clamping section reinforcement E2.

[0226] A steel bar with a yield strength Y2 of 480MPa, tensile strength T2 of 610MPa, strength-to-yield ratio of 1.27, elongation after fracture A2 of 20%, diameter D2 of 20mm and length L2 of 190mm at room temperature was selected as the clamping section steel bar E2.

[0227] S13b. The surface of the test section steel bar E1 shall be notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0228] S14b. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0229] During welding, low-alloy steel welding rods were used, with a tensile strength (T4) of 590 MPa at room temperature. The test section reinforcement E1 was butted with the clamping section reinforcement E2 at both ends, using E1 and E2 as main reinforcements. The test section reinforcement E1 and clamping section reinforcement E2 were arranged coaxially, with the end face distance between them controlled at 2-3 mm. The splice reinforcement E3 served as the welding support structure, and double-sided splice welding was performed at the joint between the test section reinforcement E1 and the clamping section reinforcement E2. The welding current was 140 A and the voltage was 25 V. No heat treatment was performed after welding.

[0230] S15b. According to GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks, the surface of E1 is notched to obtain steel bar specimens.

[0231] Furthermore, a low-temperature tensile test was conducted on the aforementioned specimens at a test temperature of -100℃. The specific steps are as follows: S21b. Sample preparation The reinforcing bar specimens are prepared according to the aforementioned method. Welded joints are formed at both ends of the test section reinforcing bar E1, and the portion of the reinforcing bar specimen located between the two welded joints is the effective test section.

[0232] S22b. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0233] The height H of the inner cavity of the test chamber used in the low-temperature tensile test is 400 mm.

[0234] S23b. The test chamber is cooled from the outside using liquid nitrogen to -100℃ and then kept at that temperature for 21 minutes.

[0235] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once all three devices have measured temperatures of -100℃±1℃.

[0236] S24b. Set the tensile rate v1 before yielding to 2.88 mm / min and the tensile rate v2 after yielding to 19.2 mm / min. Perform tensile testing.

[0237] However, during the tensile test, the specimen fractured at the yield stage, and the fracture occurred at the E2 section of the clamped reinforcement. This low-temperature tensile test was therefore invalid.

[0238] Comparative Example 3 The method for preparing the steel reinforcement specimens in this comparative example includes the following steps: S11b. Take the reinforcing bar to be tested, and cut out a test section E1 with a length L1 of 400mm and a bracing bar E3 with a length L3 of 140mm from the reinforcing bar to be tested.

[0239] According to GB / T228.1-2021 Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Method, the yield strength Y1 and tensile strength T1 of the tested steel bar at room temperature were measured to be 454 MPa and 577 MPa, respectively.

[0240] The diameter of the reinforcing bar to be tested was measured to be 20mm. Therefore, the diameter D1 of the test section reinforcing bar E1 and the diameter D3 of the splice reinforcing bar E3 are both 20mm.

[0241] S12b. Select the clamping section reinforcement E2.

[0242] A steel bar with a yield strength Y2 of 568 MPa, tensile strength T2 of 798 MPa, strength-to-yield ratio of 1.40, elongation after fracture A2 of 18%, diameter D2 of 20 mm, and length L2 of 190 mm at room temperature was selected as the clamping section steel bar E2.

[0243] S13b. The surface of the test section steel bar E1 shall be notched in accordance with GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks.

[0244] S14b. Welding Before welding, the end faces of the test section steel bar E1 and the clamping section steel bar E2 are mechanically polished to ensure that the end faces are smooth and flat.

[0245] During welding, low-alloy steel welding rods were used, with a tensile strength (T4) of 590 MPa at room temperature. The test section reinforcement E1 was butted with the clamping section reinforcement E2 at both ends, using E1 and E2 as main reinforcements. The test section reinforcement E1 and clamping section reinforcement E2 were arranged coaxially, with the end face distance between them controlled at 2-3 mm. The splice reinforcement E3 served as the welding support structure, and double-sided splice welding was performed at the joint between the test section reinforcement E1 and the clamping section reinforcement E2. The welding current was 140 A and the voltage was 25 V. No heat treatment was performed after welding.

[0246] S15b. According to GB / T 228.3-2019 Metallic materials, tensile testing - Part 3: Low temperature test method and YB / T4641-2018 Low temperature steel bars for liquefied natural gas storage tanks, the surface of E1 is notched to obtain steel bar specimens.

[0247] Furthermore, a low-temperature tensile test was conducted on the aforementioned specimens at a test temperature of -100℃. The specific steps are as follows: S21b. Sample preparation The reinforcing bar specimens are prepared according to the aforementioned method. Welded joints are formed at both ends of the test section reinforcing bar E1, and the portion of the reinforcing bar specimen located between the two welded joints is the effective test section.

[0248] S22b. The clamps of the tensile testing machine clamp the clamping sections E2 at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber.

[0249] The height H of the inner cavity of the test chamber used in the low-temperature tensile test is 400 mm.

[0250] S23b. The test chamber is cooled from the outside using liquid nitrogen to -100℃ and then kept at that temperature for 21 minutes.

[0251] The test chamber is equipped with three temperature testing devices, which are evenly spaced along the length of the test section steel bar E1, with each device 2mm away from the surface of the steel bar E1. The temperature holding time begins once all three devices have measured temperatures of -100℃±1℃.

[0252] S24b. Set the tensile rate v1 before yielding to 2.88 mm / min and the tensile rate v2 after yielding to 19.2 mm / min. Perform tensile testing.

[0253] The fracture occurred within the effective test section of the reinforcing bar specimen. The yield strength Y of the tested section of reinforcing bar E1 at -100℃ was measured to be 540 MPa, and the tensile strength T was 718 MPa.

[0254] S25a. Open the test chamber and take out the fractured specimen. However, the length of the effective test segment after fracture cannot meet the requirements for manual measurement of the total elongation at maximum force (Agt).

[0255] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing steel bar specimens for low-temperature tensile testing, characterized in that, The preparation method includes: Take the steel bar to be tested and test its yield strength Y1 at room temperature; Select the clamping section reinforcement, wherein the clamping section reinforcement satisfies the yield strength Y2≥Y1+100×b at room temperature, and the coefficient b=0.5~1.5; A test section of the reinforcing bar is cut from the reinforcing bar to be tested, and clamping sections of the reinforcing bar are welded to both ends of the test section by bracing. The resulting reinforcing bar sample is obtained.

2. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 1, characterized in that, The preparation method further includes: The tensile strength T1 of the steel bar to be tested and the tensile strength T2 of the clamped section steel bar are tested at room temperature, satisfying T2≥T1+140×c, with coefficient c=1~2.

3. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 2, characterized in that, The preparation method further includes: Establish a correspondence between coefficients c and / or b and the test temperature T of the low-temperature tensile test, wherein the correspondence is configured such that as the test temperature T decreases, the value of coefficient b and / or the value of coefficient c increases accordingly. Based on the target value of the test temperature T, the value of coefficient c and / or the value of coefficient b are determined using the aforementioned correspondence.

4. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 3, characterized in that, The test temperature T is a Celsius value below 0°C; The correspondence includes: a first relation function c = 1 + 0.005 × |T|, and / or a second relation function b = 0.5 + 0.005 × |T|; Where |T| represents the absolute value of the test temperature T.

5. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 1, characterized in that, The preparation method further includes: The strength-to-yield ratio and elongation after fracture A2 of the clamped steel bars were tested at room temperature, and the strength-to-yield ratio was ≥1.25 and A2 was ≥15%.

6. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 1, characterized in that, The phrase "cutting the reinforcing bar to be tested as a test section, and welding clamping sections of reinforcing bar to both ends of the test section through brazing to obtain the reinforcing bar sample" includes: Test sections and bracing bars are cut from the steel bar to be tested. Clamping sections are then joined at both ends of the test section, and bracing bars are used as the welding support structure to perform bracing welding, thus obtaining the steel bar sample.

7. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 1, characterized in that, Double-sided welding is used for the rib welding.

8. A method for preparing steel bar specimens for low-temperature tensile testing, characterized in that, The preparation method includes: The inner cavity length H of the test chamber used for the low-temperature tensile test is collected; The reinforcing bars to be tested are cut into test sections and reinforcement bars respectively, and the diameter D1 of the test section is collected. The length L1 of the test section and the length L3 of the reinforcement bars are controlled to satisfy L1≤H+L3-0.5×D1. The length direction of the inner cavity of the test chamber is consistent with the length direction of the test section. Remove the clamped section of reinforcing steel; The two ends of the test section steel bar are respectively connected to clamping section steel bars, and the splice steel bars are used as welding support structures to perform splice welding to obtain the steel bar sample.

9. The method for preparing steel bar specimens for low-temperature tensile testing according to claim 8, characterized in that, L1≥200+6×D1+L3.

10. A method for low-temperature tensile testing of reinforcing bars, characterized in that, The experimental method includes the following steps: The steel bar specimen is prepared using the method for preparing steel bar specimens for low-temperature tensile testing as described in any one of claims 1 to 9, wherein the two ends of the test section steel bar form welded joints, and the portion located between the two welded joints is the effective test section. The clamps of the tensile testing machine clamp the clamping sections of the steel bar at both ends of the steel bar specimen, so that the effective test section is located inside the test chamber used for low-temperature tensile testing, and the clamps are located outside the test chamber. The test chamber is cooled to the test temperature T and then held at that temperature before tensile testing.

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