Steel structure in the use of the process will be subjected to various forms of action (load, foundation uneven settlement, temperature, etc.), so the steel should have good mechanical properties (strength, plasticity, toughness) and processing properties (cold and hot processing and welding properties), to ensure the safety and reliability of the structure. There are many kinds of steel, only a few of which meet the requirements of steel structure, such as Q235 in carbon steel, l6Mn in low alloy steel, 20 manganese vanadium steel (20MnV) for high strength bolts, etc.
1, the intensity
The strength index of steel is composed of the elastic limit (e), the yield limit (y) and the tensile limit (u). (10) the tensile strength of the rock is the maximum stress that can be sustained before the steel is destroyed. At this time, the structure loses its performance due to the large plastic deformation. The value of the 'sigma u/ sigma y' is the parameter of the steel strength reserve.
2, the plastic
The plasticity of steel generally refers to the fact that after the stress exceeds the yield point, it has significant plastic deformation without fracture. The main indexes used to measure the plastic deformation ability of steel are elongation ratio and section shrinkage ratio.
3. Cold bending performance
The cold bending performance of steel is a measure of the resistance of steel to crack when bending at room temperature produces plastic deformation. The cold bending property of steel is tested by cold bending test.
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 to measure the resistance of steel to impact load, which may lead to brittle fracture due to low temperature and stress concentration. The impact toughness index of steel is usually obtained by the impact test of standard specimen.
5. Welding performance
The welding property of steel is to obtain good welding joint under certain welding process condition. The welding performance can be divided into two kinds: the welding performance in the welding process and the welding performance in service. The welding performance in the welding process refers to the sensitivity that the weld and the metal near the weld do not produce thermal crack or cooling does not produce cooling shrinkage crack. Good welding performance refers to the fact that under certain welding process conditions, the weld metal and the adjacent base metal do not produce cracks. The welding performance in terms of service performance refers to the impact toughness at the weld seam and the ductility performance in the heat-affected zone, and requires that the mechanical properties of the steel in the weld seam and the heat-affected zone should not be lower than the mechanical properties of the base metal. In China, the welding performance test method of welding process is adopted, and the welding performance test method of usability and nature is also adopted.
There are many factors affecting the durability of steel. First is the poor corrosion resistance of steel, must take protective measures to prevent steel corrosion rust. Protective measures are: the regular maintenance of steel paint, are made of galvanized steel, with acid, alkali, salt and so on under the condition of strong corrosive medium, the use of special protective measures, such as offshore platform structure using anodic protection measures to prevent the corrosion of jacket, catheter frame fixed on zinc ingot, water electrolyte will automatically first corrosion of zinc ingot, so as to protect the function of steel jacket. Secondly, because the steel under high temperature and long-term load, its failure strength is much lower than the short-term strength, so the steel under long-term high temperature, to determine the lasting strength. Steel with the passage of time will automatically become hard, brittle, that is, "aging" phenomenon. The impact toughness of steel under low temperature load should be tested.
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