Abstract: steel is widely used and has a wide variety. According to different section shapes, it can be divided into four categories: profile, plate, pipe and metal products.
The selection of steel should not only ensure the safety and reliability of structures,no fake and inferior steel products, but also be economical and reasonable, which must be treated with caution.
The selection of steel shall be comprehensively considered according to the importance of the structure, load characteristics, connection method, working environment, stress history, steel thickness and other factors.
According to the unified standard for reliability design of building structures (gb50068-2001), structures and components can be divided into important, general and secondary categories according to their use, location and severity of damage consequences, and the corresponding safety levels are level I, level II and level III.
Different types of structures or components shall be made of different steels. Heavy industrial building structures, long-span structures, high-rise or super high-rise civil building structures and heavy duty crane beams or structures are important primary structures, and high-quality steels shall be selected;
General industrial and civil building structures belong to secondary structures, and ordinary quality steel can be selected according to the nature of work.
The framework of temporary houses and the auxiliary components in general buildings, such as ladders and railings, belong to secondary three types of structures, and poor quality steel can be selected.
The load can be divided into static load and dynamic load. High quality steel with good toughness and fatigue resistance, such as Q345 steel, shall be selected for structures directly bearing dynamic loads and structures in strong earthquake areas, and appropriate additional guarantee items shall be put forward.
Q235 steel with lower price can be selected for structural members generally bearing static load or indirect dynamic load at room temperature.
There are two connection methods of steel structure: welding and non welding. During welding, welding deformation, welding stress and other welding defects (such as undercut, air hole, crack, slag inclusion, etc.) will occur, which may lead to cracks or brittle fracture of the structure. Therefore, the material requirements of welded structure should be strict.
For example, in terms of chemical composition, the limit content of carbon, sulfur and phosphorus must be strictly controlled for welded structures, and the carbon content of non welded structures can be reduced.
4. Temperature and environment of the structure
The environmental conditions of the structure, such as temperature change and corrosive medium, have a great impact on the mechanical properties of steel.
Structural members under low temperature conditions, especially welded structures and tensile members, are prone to low-temperature cold brittle fracture failure, so calm steel with good low-temperature brittle fracture resistance should be selected.
Steel structures in corrosive media, such as steel structure plants of chemical enterprises, shall adopt corrosion-resistant steel and strengthen the rust prevention treatment of exposed steel members.
In addition, the steel of open-air structure is prone to aging, and the steel under the action of harmful medium is prone to corrosion, fatigue and fracture. Different materials should also be selected differently.
Thin steel has more rolling times and higher compression ratio;
The compression ratio of steel with large thickness is small. Therefore, the steel with large thickness not only has low strength, but also has poor plasticity, impact toughness and weldability. Therefore, the welding structure with large thickness shall be made of steel with good material.