Material requirements for steel structures

Steel structures are subjected to various types of forces during use (loads, uneven foundation settlement, temperature changes, etc.). Therefore, the steel used must possess excellent mechanical properties (strength, plasticity, and toughness) and processing properties (cold and hot working and welding properties) to ensure the safety and reliability of the structure. Many types of steel exist, but only a few meet the requirements for steel structures, such as Q235 in carbon steel, 16Mn in low-alloy steel, and 20MnV (20 manganese vanadium steel) used for high-strength bolts.

 

Performance Indicators
1. Strength
The strength indicators of steel include the elastic limit σe, yield limit σy, and tensile limit σu. Design is based on the yield strength of the steel. High yield strength can reduce the self-weight of the structure, save steel, and lower costs. The tensile strength σu is the maximum stress the steel can withstand before failure. At this point, the structure loses its usability due to significant plastic deformation, but the structure deforms significantly without collapsing, meeting the requirements for resisting rare earthquakes. The value of σu/σy can be considered a parameter of the steel's strength reserve.

 

2. Plasticity
The plasticity of steel generally refers to its ability to undergo significant plastic deformation without fracturing after the stress exceeds the yield point. The main indicators for measuring the plastic deformation capacity of steel are elongation δ and reduction of area ψ.

 

3. Cold Bending Performance
The cold bending performance of steel measures its resistance to cracking during plastic deformation caused by bending at room temperature. The cold bending performance of steel is tested by a cold bending test to determine its bending deformation performance under a specified degree of bending.

 

4. Impact Toughness
The impact toughness of steel refers to its ability to absorb mechanical kinetic energy during fracture under impact load. It is a mechanical property that measures the steel's resistance to impact loads and potential brittle fracture due to low temperature and stress concentration. The impact toughness index of steel is generally obtained through impact tests on standard specimens.

 

5. Weldability
The weldability of steel refers to its ability to produce high-quality welded joints under certain welding process conditions. Weldability can be divided into two types: weldability during the welding process and weldability in terms of service performance. Weldability during the welding process refers to the susceptibility of the weld and the metal near the weld to not produce hot cracks during welding or cooling shrinkage cracks during cooling. Good weldability means that under certain welding process conditions, neither the weld metal nor the adjacent base metal will produce cracks. Weldability in terms of performance refers to the impact toughness at the weld and the ductility in the heat-affected zone, requiring that the mechanical properties of the steel in the weld and heat-affected zone are not lower than those of the base metal. my country uses both welding process weldability testing methods and performance-based weldability testing methods.

 

6. Durability
Many factors affect the durability of steel. Firstly, steel has poor corrosion resistance, so protective measures must be taken to prevent corrosion and rusting. Protective measures include: regular painting and maintenance of the steel, using galvanized steel, and employing special protective measures in the presence of strong corrosive media such as acids, alkalis, and salts. For example, offshore platform structures use "anodic protection" measures to prevent corrosion of the jacket structure, by fixing zinc ingots to the jacket. The seawater electrolyte will automatically corrode the zinc ingots first, thus protecting the steel jacket structure. Secondly, because the fracture strength of steel under high temperature and long-term load is significantly lower than its short-term strength, the long-term strength of steel under prolonged high-temperature conditions must be determined. Steel automatically hardens and becomes brittle over time, a phenomenon known as "aging." The impact toughness of steel under low-temperature loads must also be tested.