Method for rapidly determining existence of tempered martensite structure in hot-rolled steel bar

Abstract

The invention discloses a method for quickly determining whether a tempered martensite structure exists in a hot-rolled steel bar, which comprises the following steps of: cutting a steel bar with a certain length at room temperature according to the characteristic that different metallographic structures of the steel bar are different and the conductivity of the steel bar is also different, and measuring the resistivity of the steel bar by using a resistance tester so as to judge whether the steel bar has the tempered martensite structure. And if the tempered martensite structure does not exist, directly judging that the steel bar meets the standard requirement, and if the tempered martensite structure exists, directly judging that the steel bar is unqualified. According to the detection method provided by the standard, the time from cutting to corrosion observation of each sample is usually several hours, but the result can be measured by adopting the method disclosed by the invention only in several minutes, so that the detection time of the metallographic structure of the reinforcing steel bar is greatly shortened, the working efficiency is improved, and meanwhile, the method has certain practical significance on energy conservation and emission reduction; and meanwhile, the method is beneficial supplement to the existing standard method.

Description

Technical Field

[0001] The invention relates to a method for determining tempered martensite structure, in particular to a method for quickly determining whether a hot-rolled steel bar has tempered martensite structure. Background Art

[0002] Hot-rolled ribbed steel bars, commonly known as rebars, are a type of steel widely used in construction and infrastructure construction. They have high strength, can withstand greater pressure and tension, and have good durability. They also have good processing and welding properties, and can be easily processed and installed. In addition, hot-rolled ribbed steel bars can also adopt prestressed tensioning technology to improve the crack resistance and bearing capacity of the components.

[0003] In the field of construction, buildings need to withstand the influence of various factors such as the natural environment and the use environment, so high-strength materials are needed to ensure the safety and stability of the buildings. Due to its high strength, hot-rolled ribbed steel bars can effectively improve the bearing capacity of buildings, so they are widely used in bridges, roads, high-rise buildings and other buildings. In the field of construction, the service life of materials is very important. After the hot-rolled ribbed steel bars are properly treated with anti-corrosion, their service life can reach decades or even hundreds of years, so they can effectively ensure the long-term use of buildings.

[0004] With the rapid development of China's economy, the demand for infrastructure and construction has continued to increase, which has also led to the annual growth of the output of hot-rolled ribbed steel bars. According to relevant statistics, China's output of hot-rolled ribbed steel bars has ranked first in the world for many years. In short, hot-rolled ribbed steel bars are an important construction steel material that plays an indispensable role in China's economic development. As a high-strength, durable and easy-to-construct construction steel material, it is widely used in construction projects. With the continuous improvement of construction projects' requirements for material performance, the application prospects of rebar will be broader.

[0005] At present, the national standard for hot-rolled ribbed steel bars is GB 1499.2-2024 "Steel for reinforced concrete Part 2: Hot-rolled ribbed steel bars". The standard requires that the metallographic structure of the steel bars is mainly ferrite plus pearlite, and tempered martensite should not appear on the base circle. The inspection method is: GB / T 13298-2015 "Methods for testing metal microstructures" and Appendix B of GB 1499.2-2024. The method requires that the steel bars be cut, ground, polished and corroded with nitric acid alcohol, and finally the color contrast changes of the sample cross section are observed to determine whether there is tempered martensite on the base circle of the sample. Figure 1As shown in the figure, a) is a steel bar without tempered martensite on the base circle, which meets the standard requirements; b) is a steel bar with tempered martensite on the base circle and the tempered martensite is in a closed state, which does not meet the standard requirements; c) is a semi-closed state, and it is necessary to further use metallographic method or hardness method to determine whether the edge structure is tempered martensite structure. If it is tempered martensite structure, the steel bar does not meet the standard requirements. Ferrite plus pearlite structure is as follows Figure 2 As shown, the tempered martensite structure is Figure 3 shown.

[0006] The above method can accurately determine whether the steel bar has tempered martensite structure, but the inspection process is relatively complicated and is not conducive to large-scale rapid detection. Summary of the invention

[0007] The object of the present invention is to provide a method for quickly determining whether a hot-rolled steel bar has a tempered martensite structure, which can quickly and accurately determine whether a hot-rolled steel bar has a tempered martensite structure.

[0008] The technical solution of the present invention is a method for quickly determining whether hot-rolled steel bars have tempered martensite structure. According to the characteristics that different metallographic structures of steel bars have different conductive properties, a certain length of steel bars is cut at room temperature, and the resistivity is measured by a resistance tester to determine whether the steel bars have tempered martensite structure. If no tempered martensite structure is present, the steel bars are directly determined to meet the standard requirements. If tempered martensite structure is present, the steel bars are directly determined to be unqualified.

[0009] In the aforementioned method for quickly determining whether a hot-rolled steel bar has tempered martensite structure, when the measured resistivity is greater than 0.0200Ω·mm 2 / m, it is judged that the steel bar has tempered martensite structure; when the measured resistivity is 0.0172Ω·mm 2 / m, and ≤0.0200Ω·mm 2 / m, it can be determined that the steel bar does not have tempered martensite structure and the steel bar meets the standard requirements.

[0010] In the aforementioned method for rapidly determining whether a hot-rolled steel bar has a tempered martensite structure, during the determination, the temperature of the determination space is maintained at 20°C.

[0011] In the aforementioned method for quickly determining whether a hot-rolled steel bar has a tempered martensite structure, the hot-rolled steel bar refers to hot-rolled steel bars with three strength grades of 400, 500 and 600.

[0012] Beneficial effects of the present invention: Compared with the prior art, according to the inspection method provided by the standard, the time required for each sample from cutting to corrosion observation is usually several hours, while the above-mentioned method of the present invention can measure the results in just a few minutes, which is not only conducive to greatly shortening the inspection time of the metallographic structure of the steel bar and improving work efficiency, but also has certain practical significance for energy conservation and emission reduction, and is also a beneficial supplement to the existing standard methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 These are several special forms of hot-rolled ribbed steel bars after grinding and corrosion; Figure 2 It is the ferrite plus pearlite structure of hot-rolled ribbed steel bars; Figure 3 It is the tempered martensite structure of hot rolled ribbed steel bars. DETAILED DESCRIPTION

[0014] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but they are not intended to limit the present invention.

[0015] Embodiment of the present invention: A method for quickly determining whether hot-rolled steel bars have tempered martensite structure. For the same grade of steel bars, different metallographic structures have different electrical conductivity. For example, at room temperature, the metallographic structure of the steel bars may be ferrite plus pearlite, or tempered martensite. Due to the large solid solubility, the lattice distortion of the tempered martensite structure is large, and microscopic defects such as dislocations, twins, and stacking faults are relatively more, resulting in a large resistivity of the structure, while the lattice distortion and microscopic defects of ferrite plus pearlite are relatively less, so its resistivity is also relatively low.

[0016] According to the characteristics that different metallographic structures of steel bars have different conductive properties, a certain length of steel bars is cut at room temperature, and the resistivity is measured by a resistance tester, so as to determine whether the steel bars have tempered martensite structure. If no tempered martensite structure is present, the steel bars are directly determined to meet the standard requirements. If tempered martensite structure is present, the steel bars are directly determined to be unqualified.

[0017] The specific method is as follows: 1) Use a resistance tester to measure and calculate the resistivity of hot-rolled ribbed steel bars without tempered martensite structure ρ 0. After a large number of tests, it was found that at 20°C, the resistivity of hot-rolled ribbed steel bars (including 400, 500 and 600 grades) is about 0.0172Ω·mm 2 / m.

[0018] 2) Use a resistance tester to measure and calculate the resistivity of the hot-rolled ribbed steel bar to be tested ρ1. At 20°C, the resistivity of hot-rolled ribbed steel bars with tempered martensite structure is about 0.0200Ω·mm 2 / m or above.

[0019] 3) If the resistivity of the hot-rolled ribbed steel bar is about 0.0172Ω·mm 2 / m, and not more than 0.0200Ω·mm 2 / m, it can be determined that the steel bar does not have tempered martensite structure and meets the standard requirements.

[0020] During the measurement, the temperature of the measurement space was maintained at 20°C to prevent the measured resistivity from being affected by different temperatures, which would result in a difference between the measured resistivity and the resistivity measured at 20°C.

[0021] The hot-rolled steel bars refer to hot-rolled steel bars with strength grades of 400, 500 and 600. Among them, the mass fractions of alloy elements of 400 and 500 are as follows: carbon (C) ≤ 0.25%, silicon (Si) ≤ 0.80%, manganese (Mn) ≤ 1.60%, phosphorus (P) ≤ 0.045%, sulfur (S) ≤ 0.045%, and the mass fraction requirements of the remaining elements of 600-grade steel bars are the same except that the mass fraction of carbon (C) is ≤ 0.28%. A small increase in carbon content will not cause significant changes in the metallographic structure. At the same time, the pearlite resistivity is less affected by the change in the carbon content of the steel bar, and this is also the case in actual measurements. Therefore, for steel bars whose chemical composition meets the standard requirements, the influence of chemical elements can be ignored in actual measurements.

[0022] In order to verify the effectiveness of this method, a hot-rolled ribbed steel bar with a grade of HRB400E and a nominal diameter of Φ16mm was used as an example for the experiment. There was no tempered martensite structure on the base circle of one hot-rolled ribbed steel bar. Its resistivity was measured at 20℃. The three measurement results were: 0.0170Ω·mm 2 / m, 0.0172Ω·mm 2 / m, 0.0169Ω·mm 2 / m; another hot-rolled ribbed steel bar has tempered martensite structure on its base circle, and the three measurement results are: 0205Ω·mm 2 / m, 0.0211Ω·mm 2 / m, 0.0204Ω·mm 2 / m. It can be seen that due to the presence of tempered martensite on the base circle, its resistivity has an obvious mutation. Through the change of resistivity, it can be quickly judged whether the tested steel bar meets the standard requirements.

Claims

1. A method for quickly determining whether a hot-rolled steel bar has a tempered martensite structure, characterized in that: According to the different metallographic structures of steel bars, their conductive properties are also different. A certain length of steel bars is cut at room temperature and their resistivity is measured using a resistance tester to determine whether the steel bars have tempered martensite structure. If there is no tempered martensite structure, the steel bars are directly judged to meet the standard requirements. If there is tempered martensite structure, the steel bars are directly judged to be unqualified.

2. A method for rapidly determining whether a hot-rolled steel bar has a tempered martensite structure according to claim 1, characterized in that: When the measured resistivity is greater than 0.0200Ω·mm 2 / m, it is judged that the steel bar has tempered martensite structure; when the measured resistivity is 0.0172Ω·mm 2 / m, and ≤0.0200Ω·mm 2 / m, it can be determined that the steel bar does not have tempered martensite structure and the steel bar meets the standard requirements.

3. The method for quickly determining whether a hot-rolled steel bar has a tempered martensite structure according to claim 1, characterized in that: During the measurement, the temperature of the measurement space was maintained at 20°C.

4. The method for rapidly determining whether a hot-rolled steel bar has a tempered martensite structure according to claim 1, characterized in that: The hot-rolled steel bars refer to hot-rolled steel bars with three strength grades: 400, 500 and 600.

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