Low-temperature steel test sample for tensile test and preparation method and application thereof

Abstract

This invention discloses a low-temperature steel rebar specimen for tensile testing, its preparation method, and its application, relating to the field of steel performance testing technology. The specimen includes a matrix comprising a reinforced zone, a transition zone, and a test zone. There are two reinforced zones and two transition zones, located at opposite ends of the test zone. The ends of the two transition zones furthest from the test zone are the two reinforced zones. The matrix microstructure of the reinforced and transition zones consists of ferrite and martensite, while the matrix microstructure of the test zone consists of ferrite and pearlite. By employing end-strengthening, three different zones are formed in the matrix microstructure. The reinforced and transition zones, containing a mixed phase of ferrite and martensite, exhibit improved mechanical properties compared to the test zone. This prevents the specimen from breaking outside the low-temperature chamber during low-temperature tensile testing, enabling accurate detection of the specimen's tensile properties at low temperatures.

Description

Technical Field

[0001] This invention relates to the field of steel performance testing technology, and more specifically, to a low-temperature steel bar specimen for tensile testing, its preparation method, and its application. Background Technology

[0002] Cryogenic steel reinforcement for liquefied natural gas storage tanks needs to be tested for its mechanical properties at -165°C, such as yield strength, tensile strength, and total elongation at maximum force, to verify whether it meets the manufacturer's standards. Relevant standards stipulate that the steel reinforcement cannot be processed and can only undergo direct tensile testing. Therefore, to test the mechanical properties of the steel reinforcement at extremely low temperatures, it is necessary to place it in a cryogenic chamber for testing.

[0003] During tensile testing of reinforcing bars, the ends need to be clamped before tensioning. Currently, there are two common clamping methods: one is to clamp the sample inside a cryogenic chamber. However, since the hydraulic oil in the clamping device solidifies at -165℃, it becomes inoperable, leading to clamping device failure; therefore, this method is generally not used. The other method is to pass the reinforcing bar through the cryogenic chamber and clamp the sample outside. This clamping method presents a challenge: due to cryogenic hardening, the tensile strength at low temperatures is higher than at room temperature, sometimes by hundreds of megapascals. Therefore, during the tensile test, the sample may break outside the cryogenic chamber. This means that the tensile strength detected by this method is the tensile strength of the reinforcing bar at room temperature, and cannot accurately obtain the tensile strength of the reinforcing bar under cryogenic conditions.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a low-temperature steel bar specimen for tensile testing, its preparation method, and its application.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a low-temperature steel bar specimen for tensile testing, comprising a steel bar matrix, the steel bar matrix including a reinforcing zone, a transition zone and a test zone, wherein there are two reinforcing zones and two transition zones, the two transition zones are located at both ends of the test zone, and the two ends of the two transition zones away from the test zone are two reinforcing zones.

[0008] The matrix structure of the reinforced region and the transition region both include ferrite and martensite, while the matrix structure of the test region includes ferrite and pearlite.

[0009] In an optional embodiment, the total content of ferrite and martensite in the matrix structure of the reinforced region is 25-35%, preferably 28-32%.

[0010] Preferably, the total content of ferrite and martensite in the matrix structure of the transition zone is 20-30%, more preferably 22-27%.

[0011] In an optional embodiment, the matrix in the reinforced region has a Vickers hardness increased by 50-120 HV10 and a tensile strength increased by more than 230 MPa compared to the matrix in the test region; the matrix in the transition region has a Vickers hardness increased by 1-50 HV10 and a tensile strength increased by more than 200 MPa compared to the matrix in the test region.

[0012] Preferably, the Vickers hardness of the matrix in the reinforced region is increased by 60-120 HV10 compared to the matrix in the test region, and the tensile strength is increased by more than 235 MPa; the Vickers hardness of the matrix in the transition region is increased by 10-50 HV10 compared to the matrix in the test region, and the tensile strength is increased by more than 205 MPa.

[0013] In an optional implementation, the length of the test zone is 45-60% of the total length of the reinforcing bar matrix, the length of each reinforcement zone is 18-25% of the total length of the reinforcing bar matrix, and the length of each transition zone is 1-5% of the total length of the reinforcing bar matrix.

[0014] Preferably, the length of the test zone is 47-56% of the total length of the steel reinforcement matrix, the length of each reinforcement zone is 21-25% of the total length of the steel reinforcement matrix, and the length of each transition zone is 1.5-3% of the total length of the steel reinforcement matrix.

[0015] In an optional implementation, the low-temperature steel bars include any one of the grades HRB500DW and HRB600DW.

[0016] Secondly, the present invention provides a method for preparing a low-temperature steel bar sample as described in any of the foregoing embodiments, comprising performing medium-frequency quenching heat treatment on both ends of the steel bar matrix to form a strengthening zone and a transition zone at both ends of the steel bar matrix.

[0017] In an optional embodiment, the medium-frequency quenching heat treatment includes placing both ends of the sample in a medium-frequency quenching machine, heating both ends of the steel reinforcement matrix, and then quenching.

[0018] Preferably, the heating temperature is 1045–1205°C.

[0019] Preferably, the cooling method for quenching includes room temperature water cooling or room temperature air cooling, and more preferably room temperature water cooling.

[0020] In an optional embodiment, the length of the reinforcing bar matrix entering the medium-frequency quenching machine is the length of the strengthening zone of the matrix, and the feeding speed of the reinforcing bar matrix into the medium-frequency quenching machine is 75-105 mm / min.

[0021] In an optional embodiment, the reinforcing steel base is a cylindrical base with a specification of Φ16~28mm.

[0022] Preferably, the heating temperature and injection rate of the steel reinforcement matrix for different specifications are shown in the table below:

[0023] Specifications (mm) Heating temperature (°C) Injection rate (mm / min) 16 1050±5 100±5 18 1100±5 100±5 20 1100±5 90±5 25 1150±5 80±5 28 1200±5 80±5

[0024] Thirdly, the present invention provides the application of a low-temperature steel bar sample as described in any of the foregoing embodiments or a preparation method as described in any of the foregoing embodiments in the field of steel bar performance testing.

[0025] The present invention has the following beneficial effects:

[0026] This invention provides a low-temperature steel bar specimen for tensile testing, its preparation method, and its application. By employing end-strengthening, three different zones are formed in the steel bar matrix. The strengthening zone and transition zone contain a mixed phase of ferrite and martensite, thus their mechanical properties are improved compared to the test zone. During low-temperature tensile testing, the specimen will not break outside the low-temperature chamber, and the tensile properties of the specimen at low temperatures can be accurately detected. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 Metallographic microscope image of the reinforced zone matrix of the low-temperature steel bar sample provided in Embodiment 1 of the present invention;

[0029] Figure 2 Metallographic microscope image of the transition zone matrix of the low-temperature steel bar sample provided in Embodiment 1 of the present invention;

[0030] Figure 3 Metallographic microscope image of the test area matrix of the low-temperature steel bar sample provided in Embodiment 1 of the present invention;

[0031] Figure 4 A diagram showing the results of a low-temperature tensile test on a low-temperature steel bar specimen provided in Embodiment 1 of the invention.

[0032] Figure 5 A diagram showing the results of a low-temperature tensile test on a low-temperature steel bar specimen provided as comparative example 1 of the invention.

[0033] Figure 6 The results of the low-temperature tensile test of the low-temperature steel bar specimen provided for Comparative Example 2 of the invention are shown in the figure. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0035] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0036] For low-temperature rebar testing, its mechanical properties at low temperatures need to be assessed. A common method involves passing the specimen through a cryogenic chamber, clamping it outside the chamber, and then conducting a tensile test. To obtain the tensile strength of the rebar at low temperatures, the maximum force required to clamp the specimen in the room-temperature region must be higher than the maximum force required for the low-temperature tensile test. This ensures that the specimen fractures within the cryogenic chamber, meeting the requirements for testing the tensile strength of the rebar at low temperatures. However, due to the inherent characteristics of rebar, its tensile strength increases significantly at lower temperatures compared to its room-temperature state. Therefore, tensile tests often result in the rebar fracturing in the room-temperature region, thus only detecting the tensile strength at room temperature and failing to obtain the tensile strength at low temperatures.

[0037] Currently, the main methods to solve this problem are welding thicker specimens (thicker than the base material) to both ends of the sample or forging thicker specimens at both ends to improve the mechanical properties of the sample ends and ensure that the fracture occurs within the cryogenic chamber. However, the welding method requires the welded specimen to be compatible with the material of the sample being tested, and also requires advanced welding skills to ensure the smooth conduct of the tensile test; otherwise, issues such as weld detachment may occur, failing to meet the requirements for testing the tensile strength of steel bars at low temperatures. The forging method, on the other hand, generates a large number of dislocations during forging, leading to cryogenic brittle fracture at or near the forged portion, which can also cause test failure. Therefore, the inventors have creatively proposed the following solution to address the aforementioned technical challenges.

[0038] In a first aspect, the present invention provides a low-temperature steel bar specimen for tensile testing, comprising a steel bar matrix, the steel bar matrix including a reinforcing zone, a transition zone and a test zone, wherein there are two reinforcing zones and two transition zones, the two transition zones are located at both ends of the test zone, and the two ends of the two transition zones away from the test zone are two reinforcing zones.

[0039] The matrix structure of the reinforced region and the transition region both include ferrite and martensite, while the matrix structure of the test region includes ferrite and pearlite.

[0040] By forming three different zones in the matrix structure, the mechanical properties of the reinforced zone and the transition zone are improved compared to the test zone because they contain a mixed phase of ferrite and martensite. When performing low-temperature tensile tests, the specimen will not break outside the low-temperature chamber, and the tensile properties of the specimen at low temperature can be accurately detected.

[0041] In an optional embodiment, the total content of ferrite and martensite in the matrix structure of the reinforced region is 25-35%, preferably 28-32%.

[0042] Preferably, the total content of ferrite and martensite in the matrix structure of the transition zone is 20-30%, more preferably 22-27%.

[0043] By controlling the matrix composition content in the reinforced zone and the transition zone, the mechanical properties of the steel reinforcement matrix can be controlled, resulting in a significant improvement in the mechanical properties at both ends of the matrix test zone. At the same time, in order to avoid a large difference in mechanical properties between the reinforced zone and the test zone, which could lead to fracture between the two zones, the matrix content in the transition zone is controlled to control the mechanical properties of the transition zone and ensure that the sample will not fracture within the normal temperature range.

[0044] In an optional embodiment, the matrix in the reinforced region has a Vickers hardness increased by 50-120 HV10 and a tensile strength increased by more than 230 MPa compared to the matrix in the test region; the matrix in the transition region has a Vickers hardness increased by 1-50 HV10 and a tensile strength increased by more than 200 MPa compared to the matrix in the test region.

[0045] Preferably, the Vickers hardness of the matrix in the reinforced region is increased by 60-120 HV10 compared to the matrix in the test region, and the tensile strength is increased by more than 235 MPa; the Vickers hardness of the matrix in the transition region is increased by 10-50 HV10 compared to the matrix in the test region, and the tensile strength is increased by more than 205 MPa.

[0046] In an optional implementation, the length of the test zone is 45-60% of the total length of the reinforcing bar matrix, the length of each reinforcement zone is 18-25% of the total length of the reinforcing bar matrix, and the length of each transition zone is 1-5% of the total length of the reinforcing bar matrix.

[0047] Preferably, the length of the test zone is 47-56% of the total length of the steel reinforcement matrix, the length of each reinforcement zone is 21-25% of the total length of the steel reinforcement matrix, and the length of each transition zone is 1.5-3% of the total length of the steel reinforcement matrix.

[0048] In an optional implementation, the low-temperature steel bars include those with designations such as HRB500DW and HRB600DW.

[0049] Secondly, the present invention provides a method for preparing a low-temperature steel bar sample as described in any of the foregoing embodiments, comprising performing medium-frequency quenching heat treatment on both ends of the steel bar matrix to form a strengthening zone and a transition zone at both ends of the matrix.

[0050] When existing technologies use welding or forging processes to treat the reinforcing steel matrix, welding requires the welded sample to be compatible with the material of the test specimen, and also requires advanced welding skills; otherwise, issues such as sample detachment may occur, failing to meet the requirements for testing the tensile strength of reinforcing steel at low temperatures. Forging, on the other hand, generates numerous dislocations, leading to low-temperature brittle fracture in or near the forged portion, also resulting in test failure.

[0051] This invention creatively proposes a method to treat the steel reinforcement matrix to directly improve its mechanical properties. Specifically, this is achieved by heat-treating both ends of the steel reinforcement matrix followed by cooling and quenching. By directly controlling the phase transformation at both ends of the steel reinforcement matrix, a mixed phase of martensite and ferrite is formed, which improves the Vickers hardness and tensile strength of the sample, providing a material basis for tensile fracture in a low-temperature chamber. Furthermore, this method only treats the matrix itself, without involving additional welding materials, simplifying process control, resulting in a matrix with good mechanical properties and reducing the likelihood of low-temperature brittle fracture.

[0052] In an optional embodiment, the medium-frequency quenching heat treatment includes placing both ends of the sample in a medium-frequency quenching machine, heating both ends of the reinforcing steel matrix, and then quenching. Heating both ends of the sample separately can be achieved by using one medium-frequency quenching machine to treat one end of the sample first, forming a strengthening zone and a transition zone, and then treating the other end; alternatively, two medium-frequency quenching machines can be used to treat both ends of the sample simultaneously.

[0053] Preferably, the heating temperature is 1045–1205°C.

[0054] Preferably, the quenching cooling method includes room temperature water cooling or room temperature air cooling, more preferably room temperature water cooling. Specifically, the quenching cooling process involves heating both ends of the sample to the target temperature, i.e., any target value between 1045 and 1205°C, and then starting to cool both ends of the sample until the sample is at room temperature.

[0055] In an optional implementation, the insertion length of the reinforcing steel matrix into the medium-frequency quenching machine is the length of the reinforced zone of the matrix. The reinforced zone is directly heated using the medium-frequency quenching machine to enhance its mechanical properties. During the heating process, due to heat diffusion, the matrix outside the reinforced zone is also heated simultaneously. This area, which is not directly heated but is heated by residual heat, is the transition zone. The transition zone is shorter than the reinforced zone, but because it is adjacent to the heated area of ​​the reinforced zone, its mechanical properties decrease linearly from the reinforced zone to the test zone. Therefore, the transition zone also possesses a certain level of mechanical strength, preventing matrix fracture in the transition zone.

[0056] In an optional implementation, the feed rate of the reinforcing steel matrix into the medium-frequency quenching machine is 75–105 mm / min.

[0057] In an optional embodiment, the reinforcing steel base is a cylindrical base with a specification of Φ16~28mm.

[0058] Preferably, the heating temperature and injection speed of the steel reinforcement matrix of different specifications are shown in the table below.

[0059] Table 1 Heating temperature and injection rate of steel reinforcement matrix for different specifications

[0060] Specifications (mm) Heating temperature (°C) Injection rate (mm / min) 16 1050±5 100±5 18 1100±5 100±5 20 1100±5 90±5 25 1150±5 80±5 28 1200±5 80±5

[0061] The above only lists the relevant parameters for the processing of steel bar samples of several common specifications. In the actual preparation process, the specifications and processing parameters of steel bar samples are not limited to the data listed in the table above.

[0062] Thirdly, the present invention provides the application of a low-temperature steel bar sample as described in any of the foregoing embodiments or a preparation method as described in any of the foregoing embodiments in the field of steel bar performance testing.

[0063] Example 1

[0064] This embodiment provides a method for preparing low-temperature steel bar specimens for tensile testing, including the following steps:

[0065] S1. Take a low-temperature steel bar sample with grade HRB500DW, Φ16mm, sample length 1060mm, effective height of low-temperature chamber 600mm, then the test area matrix length of the sample is about 560mm, the length of each strengthening zone is about 230mm, and the length of each transition zone is about 20mm.

[0066] S2. One end of the sample is clamped onto a medium-frequency quenching machine with a heating temperature of 1050℃. The sample enters the machine at a speed of 100mm / min for rapid heating, with a total length of 230mm, representing the length of the reinforced zone. Due to the thermal diffusion effect of the medium-frequency quenching machine, the sample feeding is stopped after 230mm. However, the matrix between 230-250mm is also heated by the residual heat of the machine and thus has a relatively high temperature. Immediately after feeding, the sample is quenched with room temperature water until its temperature returns to room. This process simultaneously quenches the high-temperature reinforced zone and the relatively high-temperature transition zone, resulting in a sample with both a reinforced zone and a transition zone at one end.

[0067] S3. Clamp the other end of the sample onto the medium-frequency quenching machine. The heating temperature of the medium-frequency quenching machine is 1050℃. The sample enters the medium-frequency quenching machine at a speed of 100mm / min for rapid heating. The total length of the sample entering the medium-frequency quenching machine is 230mm. After the sample is injected, immediately cool it with room temperature water until the sample temperature returns to room temperature, thereby obtaining a sample with both ends having a strengthening zone and a transition zone. The sample preparation is complete.

[0068] The low-temperature steel bar sample prepared in this embodiment was placed under a scanning electron microscope, and the microstructure of the strengthening zone, transition zone, and test zone of the sample was observed sequentially to obtain the following results: Figures 1-3 The results are shown. Among them, Figure 1 The image shows the microstructure of the matrix in the reinforced region. The matrix in the reinforced region contains martensite and ferrite phases, and the content of martensite and ferrite is relatively high. Figure 2 The image shows the microstructure of the matrix in the transition zone. The transition zone matrix contains martensite and ferrite phases, and the content of martensite and ferrite is reduced compared to the strengthening zone. Figure 3 The image shows the microstructure of the matrix in the test area. Pearlite and ferrite phases are present in the matrix, with almost no martensite observed. This demonstrates that the method in this embodiment effectively enhances the mechanical properties at both ends of the specimen, which is beneficial for low-temperature tensile testing.

[0069] Example 2

[0070] This embodiment provides a method for preparing low-temperature steel bar specimens for tensile testing, including the following steps:

[0071] S1. Take a low-temperature steel bar sample with grade HRB500DW, Φ18mm, sample length 1060mm, effective height of low-temperature chamber 600mm, then the test area matrix length of the sample is about 560mm, the length of each strengthening zone is about 230mm, and the length of each transition zone is about 20mm.

[0072] S2. Clamp one end of the sample onto the medium-frequency quenching machine. The heating temperature of the medium-frequency quenching machine is 1100℃. The sample enters the medium-frequency quenching machine at a speed of 100mm / min for rapid heating. The total length of the sample entering the medium-frequency quenching machine is 230mm. After the sample is fed, immediately use room temperature water to cool and quench until the temperature of the sample returns to room temperature, thus obtaining a sample with a strengthening zone and a transition zone at one end.

[0073] S3. The other end of the sample is processed according to step S2 to obtain a sample with a strengthening zone and a transition zone at both ends. The sample preparation is complete.

[0074] Example 3

[0075] This embodiment provides a method for preparing low-temperature steel bar specimens for tensile testing, including the following steps:

[0076] S1. Take a low-temperature steel bar sample with grade HRB500DW, Φ20mm, sample length 1060mm, effective height of low-temperature chamber 600mm, then the test area matrix length of the sample is about 560mm, the length of each strengthening zone is about 230mm, and the length of each transition zone is about 20mm.

[0077] S2. Clamp one end of the sample onto the medium-frequency quenching machine. The heating temperature of the medium-frequency quenching machine is 1100℃. The sample enters the medium-frequency quenching machine at a speed of 90mm / min for rapid heating. The total length of the sample entering the medium-frequency quenching machine is 230mm. After the sample is fed, immediately use room temperature water to cool and quench until the temperature of the sample returns to room temperature, thus obtaining a sample with a strengthening zone and a transition zone at one end.

[0078] S3. The other end of the sample is processed according to step S2 to obtain a sample with a strengthening zone and a transition zone at both ends. The sample preparation is complete.

[0079] Example 4

[0080] This embodiment provides a method for preparing low-temperature steel bar specimens for tensile testing, including the following steps:

[0081] S1. Take a low-temperature steel bar sample with grade HRB500DW, Φ25mm, sample length 1060mm, effective height of low-temperature chamber is 600mm, then the test area matrix length of the sample is about 560mm, the length of each strengthening zone is about 230mm, and the length of each transition zone is about 20mm.

[0082] S2. Clamp one end of the sample onto the medium-frequency quenching machine. The heating temperature of the medium-frequency quenching machine is 1150℃. The sample enters the medium-frequency quenching machine at a speed of 80mm / min for rapid heating. The total length of the sample entering the medium-frequency quenching machine is 230mm. After the sample is fed, immediately use room temperature water to cool and quench until the temperature of the sample returns to room temperature, thus obtaining a sample with a strengthening zone and a transition zone at one end.

[0083] S3. The other end of the sample is processed according to step S2 to obtain a sample with a strengthening zone and a transition zone at both ends. The sample preparation is complete.

[0084] Example 5

[0085] This embodiment provides a method for preparing low-temperature steel bar specimens for tensile testing, including the following steps:

[0086] S1. Take a low-temperature steel bar sample with grade HRB500DW, Φ28mm, sample length 1060mm, effective height of low-temperature chamber is 600mm, then the test area matrix length of the sample is about 560mm, the length of each strengthening zone is about 230mm, and the length of each transition zone is about 20mm.

[0087] S2. Clamp one end of the sample onto the medium-frequency quenching machine. The heating temperature of the medium-frequency quenching machine is 1200℃. The sample enters the medium-frequency quenching machine at a speed of 80mm / min for rapid heating. The total length of the sample entering the medium-frequency quenching machine is 230mm. After the sample is fed, immediately use room temperature water to cool and quench until the temperature of the sample returns to room temperature, thus obtaining a sample with a strengthening zone and a transition zone at one end.

[0088] S3. The other end of the sample is processed according to step S2 to obtain a sample with a strengthening zone and a transition zone at both ends. The sample preparation is complete.

[0089] Comparative Example 1

[0090] This comparative example provides a method for preparing a low-temperature steel bar specimen for tensile testing. The steps are similar to those in Example 1, except that the heating temperature of the medium-frequency quenching machine is 800℃.

[0091] Comparative Example 2

[0092] This comparative example provides a method for preparing a low-temperature steel bar specimen for tensile testing. The steps are similar to those in Example 1, except that the sample feeding speed into the medium-frequency quenching machine is 150 mm / min.

[0093] Comparative Example 3

[0094] This comparative example provides a method for preparing a low-temperature steel bar specimen for tensile testing. The steps are similar to those in Example 1, except that only heating is performed without quenching.

[0095] Comparative Example 4

[0096] This comparative example provides a method for preparing a low-temperature steel bar specimen for tensile testing. The steps are similar to those in Example 1, except that the heating temperature of the medium-frequency quenching machine is 1300℃.

[0097] Experimental Example 1

[0098] The low-temperature steel bar samples prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to mechanical property testing. The tensile strength was the tensile strength of the test area under normal temperature conditions, specifically tested using the GB / T228.1-2021 method, and the Vickers hardness was tested using the GB / T4340.1-2009 method. The results are shown in Table 2.

[0099] Table 2 Mechanical properties of low-temperature steel reinforcement specimens

[0100]

[0101] As shown in Table 2, the room temperature tensile strength of all steel bars in the test areas of the examples and comparative examples is the same. Since the sample preparation is for low temperature tensile testing, the length of the reinforced zone and the transition zone only needs to meet the clamping length required for tensile testing. Therefore, the tensile strength of the reinforced zone and the transition zone with shorter length and very short length can be tested by the following low temperature tensile test to see if they meet the standard.

[0102] The low-temperature steel bar specimens prepared in Examples 1-5 and Comparative Examples 1-4 were subjected to low-temperature tensile tests, specifically according to the YB / T4641-2018 method. The temperature of the low-temperature chamber used in the experiment was -165℃. The location of tensile fracture is shown in Table 3 below.

[0103] Table 3. Low-temperature tensile properties of the steel reinforcement specimens.

[0104]

[0105]

[0106] As shown in Tables 2 and 3, the present invention, by employing end-strengthening, significantly improves the mechanical properties of the specimen by heat-treating both ends of the matrix followed by cooling and quenching. This directly controls the phase transformation at both ends of the matrix, forming a mixed phase of martensite and ferrite, which increases the Vickers hardness and tensile strength of the specimen, providing a material basis for tensile fracture in a low-temperature chamber. Furthermore, this method only treats the matrix itself, without involving additional welding materials, simplifying process control, resulting in a matrix with good mechanical properties and reducing the likelihood of low-temperature brittle fracture.

[0107] Furthermore, according to Figure 4 , Figure 5 and Figure 6 Records show that... Figure 4 In Embodiment 1 of this invention, the steel bar sample was broken during testing in a low-temperature chamber, which allows for the detection of the mechanical properties of the steel bar at low temperatures. Figure 5 The steel bar sample was broken outside the low-temperature chamber, meaning that the low-temperature mechanical properties of the reinforced zone were still poor and insufficient to provide enough tensile support, making it impossible to detect the mechanical properties of the steel bar at low temperatures. Figure 6 The steel bar sample was broken at the joint in the low-temperature chamber, which means that the mechanical properties of the transition zone are poor and insufficient to provide enough tensile support, so the mechanical properties of the steel bar at low temperature cannot be detected.

[0108] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing low-temperature steel bar specimens for tensile testing, characterized in that, The low-temperature steel bar sample includes a steel bar matrix, which includes a strengthening zone, a transition zone, and a test zone. There are two strengthening zones and two transition zones. The two transition zones are located at both ends of the test zone, and the two ends of the two transition zones away from the test zone are the two strengthening zones. The matrix structure of both the reinforced region and the transition region includes ferrite and martensite, with the total content of ferrite and martensite in the matrix structure of the reinforced region being 25-35%; the matrix structure of the test region includes ferrite and pearlite, with the total content of ferrite and martensite in the matrix structure of the transition region being 20-30%. The length of the test zone is 45-60% of the total length of the reinforcing steel matrix, the length of each reinforcement zone is 18-25% of the total length of the reinforcing steel matrix, and the length of each transition zone is 1-5% of the total length of the reinforcing steel matrix. The method for preparing the low-temperature steel bar sample includes: performing medium-frequency quenching heat treatment on both ends of the steel bar matrix to form a strengthening zone and a transition zone at both ends of the steel bar matrix; The medium-frequency quenching heat treatment includes placing both ends of the sample in a medium-frequency quenching machine, heating both ends of the steel reinforcement matrix, and then quenching. The length of the reinforcing bar matrix entering the medium-frequency quenching machine is the length of the reinforcing bar matrix's strengthening zone. The injection speed of the reinforcing bar matrix into the medium-frequency quenching machine is 75~105mm / min; the heating temperature is 1045~1205℃.

2. The preparation method according to claim 1, characterized in that, The total content of ferrite and martensite in the matrix of the reinforced region is 28-32%; The total content of ferrite and martensite in the matrix structure of the transition zone is 22-27%.

3. The preparation method according to claim 1, characterized in that, The matrix in the reinforced region has a Vickers hardness that is 50-120 HV10 higher than that in the test region, and a tensile strength that is more than 230 MPa higher; the matrix in the transition region has a Vickers hardness that is 1-50 HV10 higher than that in the test region, and a tensile strength that is more than 200 MPa higher.

4. The preparation method according to claim 3, characterized in that, The Vickers hardness of the matrix in the reinforced region is 60-120 HV10 higher than that of the matrix in the test region, and the tensile strength is increased by more than 235 MPa; the Vickers hardness of the matrix in the transition region is 10-50 HV10 higher than that of the matrix in the test region, and the tensile strength is increased by more than 205 MPa.

5. The preparation method according to claim 1, characterized in that, The length of the test zone is 47-56% of the total length of the reinforcing steel matrix, the length of each of the reinforcement zones is 21-25% of the total length of the reinforcing steel matrix, and the length of each of the transition zones is 1.5-3% of the total length of the reinforcing steel matrix.

6. The preparation method according to any one of claims 2 to 5, characterized in that, Low-temperature steel bars include any one of the grades HRB500DW and HRB600DW.

7. The preparation method according to claim 1, characterized in that, The cooling methods for quenching include room temperature water cooling or room temperature air cooling.

8. The preparation method according to claim 1, characterized in that, The steel reinforcement matrix is ​​cylindrical with a specification of Φ16~28mm.

9. The preparation method according to claim 8, characterized in that, The heating temperature and injection rate of steel reinforcement substrates of different specifications are shown in the table below; 。 10. The application of the preparation method as described in any one of claims 1 to 9 in the field of steel bar performance testing.

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