Introduction

Steel has good plasticity and toughness at room temperature, but as the temperature decreases, the plasticity and toughness of steel continuously deteriorate, and the low-temperature mechanical properties of steel are related to its material, cross-sectional shape, and thickness. With the continuous improvement of power grid construction, more and more ultra-high voltage transmission lines must pass through cold regions, especially in Northeast China, where winter temperatures are low and last for a long time, and the annual extreme minimum temperature in some areas can reach -45℃ or even lower. Since transmission line towers are exposed to the atmospheric environment, they are susceptible to static, dynamic, and impact loads such as icing, strong winds, and conductor de-icing. If the design and construction are improper, ultra-high voltage transmission lines operating in cold regions are prone to low-temperature brittle fracture accidents of angle steel. According to incomplete statistics, from 1988 to 2013, there were dozens of tower collapse accidents caused by low-temperature environments in China. For ease of processing and transportation, the main components of transmission towers are usually made of multiple sections of angle steel connected by bolts.  The bolt holes are mostly processed using punching. During the punching process of bolt holes, microcracks are easily generated around the hole wall, forming crack sources. Once the temperature drops below the ductile-brittle transition temperature of the angle steel, under external load, these microcracks can easily propagate, leading to the failure of bolt connections and transmission line tower collapse accidents. The main components are critical parts of transmission towers; once low-temperature brittle fracture occurs, it will inevitably lead to the collapse of the entire tower, thus endangering the safe and stable operation of the entire power system. Therefore, studying the low-temperature mechanical properties of the angle steel used in transmission line towers, preventing low-temperature brittle fracture of the tower structure, and providing a basis for material selection are of great significance for ultra-high voltage transmission line projects.

1. Experiment Overview

This experiment mainly studies the low-temperature mechanical properties of Q345B and Q420C main angle steel and their welded joints. The standards referenced for low-temperature tensile testing are GB/T 228-2002 "Tensile Testing Method for Metallic Materials at Room Temperature" and GB/T 13239-2006 "Low-Temperature Tensile Testing Method for Metallic Materials". The standard used for low-temperature impact testing is GB/T 2009-2007 "Charpy Pendulum Impact Testing Method for Metallic Materials". The standards referenced for sample processing and sampling are GB/T 2975-1998 "Sampling Location and Sample Preparation for Mechanical Performance Testing of Steel and Steel Products".

The geometric dimensions of the samples are shown in Figure 1. The results after the experiment are shown in Figure 2.

The material types for the low-temperature tensile test include Q345B angle steel, Q345B welded joints, Q420C angle steel, and Q420C welded joints. The selected main angle steel specifications are L125×12, L140×14, and L160×16 (denoted as 12, 14, and 16 mm, respectively), and the selected welding plate thicknesses are 12, 14, and 16 mm. The test temperatures included room temperature (20℃), -10℃, -20℃, and -45℃, and a total of 144 tensile tests were completed.

The steel types, angle steel models, and welded plate thicknesses for the low-temperature impact tests were the same as those for the tensile tests. The temperatures used during the tests were room temperature (20℃), -10℃ (Q345B angle steel), -20℃, -45℃, and -60℃ (Q345B welded joints, Q420C angle steel and its welded joints), and a total of 144 impact tests were completed. The specific test list is shown in Table 1.

2. Tensile Test Results Analysis

2.1 Strength Indicators

Under low-temperature conditions, the tensile strength of Q345B angle steel, Q345B welded joints, Q420C angle steel, and Q420C welded joints all increased. However, the tensile strength did not increase monotonically with decreasing temperature. The tensile strength values varied significantly with different steel thicknesses, but among the three thickness specifications of 12-16 mm, the tensile strength was not necessarily higher for thinner steel; there was no definitive relationship between thickness and tensile strength. The base metal of both Q345B and Q420C had higher tensile strength than the welded joints, with a difference of 50-100 MPa, indicating that welding reduces the tensile strength of the steel.

In the extremely cold region of -45℃, the yield strength of the above four types of steel increased, but the yield strength did not increase monotonically with decreasing temperature. The yield strength of Q420C welded joints was lower than that of its base metal, while the yield strength of Q345B welded joints was higher than that of its base metal, indicating that the welding performance of Q345B is superior to that of Q420C under low-temperature conditions.

Under low-temperature conditions, the yield ratio of the steel increased slightly, and the material's resistance to deformation weakened. At the same time, the yield ratio of both Q345B and Q420C welded joints was higher than that of the base metal, indicating that the welded joints have weaker resistance to deformation and are more prone to brittle fracture at low temperatures. 2.2 Plasticity Index

Under low-temperature conditions, the elongation after fracture of Q345B angle steel decreases slightly, indicating poorer plasticity. However, the elongation after fracture of Q345B welded joints, Q420C angle steel, and some thicknesses of Q420C welded joints slightly increases or shows inconsistent trends at low temperatures. The plastic deformation ability of Q420C angle steel at low temperatures is superior to that of Q345B angle steel, while the welding performance of Q345B material at low temperatures is superior to that of Q420C.

3 Impact Test Results Analysis

The relationship between impact energy and temperature was analyzed using the Boltzmann function. The results show that the impact energy of both types of angle steel and welded joints decreases with decreasing temperature, and after reaching a certain temperature point, the impact energy value decreases rapidly with further decrease in temperature.

The ductile-brittle transition temperature (t₀) and other parameters were obtained by fitting the Boltzmann function for different materials and thicknesses. The average ductile-brittle transition temperatures for the three different thicknesses of the same material were calculated, resulting in ductile-brittle transition temperatures of -2.59℃, -15.28℃, -32.33℃, and -6.76℃ for Q345B angle steel, Q345B welded joints, Q420C angle steel, and Q420C welded joints, respectively. Clearly, the ability of Q420C angle steel to resist low-temperature brittle fracture is far superior to that of Q345B angle steel. At the same time, the low-temperature brittle fracture resistance of Q345B welded joints is stronger than that of the base metal, while the low-temperature brittle fracture resistance of Q420C welded joints is far lower than that of its base metal.

In the worst-case scenario, at a temperature of -45℃, only Q420C angle steel meets the specification requirement of impact energy ≥ 34J.

4 Conclusions

1) In high-cold regions at -45℃, low temperatures increase the tensile strength and yield strength of Q345B angle steel, Q345B welded joints, Q420C angle steel, and Q420C welded joints. The tensile strength of steel of different thicknesses varies under low-temperature conditions, but it is not the case that thinner steel always has higher tensile strength. 2) The ductile-brittle transition temperatures for Q345B angle steel, Q345B welded joints, Q420C angle steel, and Q420C welded joints are -2.59℃, -15.28℃, -32.33℃, and -6.76℃, respectively. Q420C angle steel has significantly better resistance to low-temperature brittle fracture than Q345B angle steel. In power transmission towers located in extremely cold regions at -45℃, using Q345B angle steel is unsafe, while Q420C angle steel can meet the design requirements.

3) Welding reduces the tensile strength and yield strength of Q420C angle steel, increasing the yield ratio. Under low-temperature conditions, welding significantly reduces the resistance of Q420C angle steel to low-temperature brittle fracture. Therefore, in power transmission towers located in extremely cold regions at -45℃, welding of Q420C should be avoided as much as possible.