Material requirements for steel structures

The steel structure will be subjected to various forms of action (load, uneven settlement of the foundation, temperature, etc.) during use, so the steel should have good mechanical properties (strength, plasticity, toughness) and processing properties (hot and cold processing and welding performance) to ensure the safety and reliability of the structure. There are many kinds of steel, and only a few kinds meet the requirements of steel structure, such as Q235 in carbon steel, l6Mn in low alloy steel, and 20 manganese vanadium steel (20MnV) for high-strength bolts.

Performance index

1. Strength

The strength index of steel is composed of elastic limit σe, yield limit σy, and tensile limit σu. The design is based on the yield strength of steel. High yield strength can reduce the weight of the structure, save steel, and reduce the cost. Tensile strength σu is the maximum stress that steel can withstand before failure. At this time, the structure loses its performance due to large plastic deformation, but the structure deformation is large and does not collapse, which meets the requirements of the structure to resist rare earthquakes. The magnitude of the 'σu/σy value can be regarded as the parameter of the steel strength reserve.

2. Plasticity

The plasticity of steel generally refers to the property of significant plastic deformation without fracture after the stress exceeds the yield point. The main indexes to measure the plastic deformation ability of steel are elongation δ and section shrinkage ψ.

3, cold bending performance

The cold bending property of steel is a measure of the resistance of steel to crack when the plastic deformation is produced by bending process at normal temperature. The cold bending performance of steel is to test the bending deformation performance of steel under specified bending degree by cold bending experiment.

4. Impact toughness

The impact toughness of steel refers to the ability of steel to absorb mechanical kinetic energy during fracture under impact load. It is a mechanical property that measures the resistance of steel to impact load cutting and may cause brittle fracture due to low temperature and stress concentration. Generally, the impact toughness index of steel is obtained by impact test of standard specimens.

5, welding performance

The weldability of steel refers to the welding joint with good performance obtained under certain welding process conditions. The welding performance can be divided into two kinds: welding performance in the welding process and welding performance in the use of the welding performance. The welding performance in the welding process refers to the sensitivity that the weld and the metal near the weld do not produce hot cracks or cooling shrinkage cracks during the welding process. Good welding performance means that under certain welding process conditions, the weld metal and the nearby base metal do not crack. The welding performance in the use of performance refers to the impact toughness of the weld and the ductility of the heat affected zone, requiring that the mechanical properties of the weld and the steel in the heat affected zone are not lower than the mechanical properties of the base material. In our country, the welding performance test method of welding process is adopted, and the welding performance test method of serviceability is also adopted.

6. Durability

There are many factors that affect the durability of steel. First of all, the corrosion resistance of steel is poor, and protective measures must be taken to prevent steel corrosion and rust. Protective measures include: regular paint maintenance of steel, the use of galvanized steel, in the presence of acid, alkali, salt and other strong corrosive medium conditions, the use of special protective measures, such as the offshore platform structure using "anode protection" measures to prevent jacket corrosion, fixed on the jacket zinc ingots, seawater electrolyte will automatically first corrosion zinc ingots, so as to protect the steel jacket function. Secondly, because the steel under high temperature and long-term load, its failure strength is less than the short-term strength, so for the steel under long-term high temperature action, it is necessary to measure the lasting strength. Steel will automatically harden and become brittle over time, that is, "aging" phenomenon. The impact toughness of steel under low temperature load should be tested.