According to the “Uniform Standards for the Design of Building Structures for Reliability” (GB50068-2001), structures and members can be divided into three categories: important, general and minor according to their use, location and severity of damage consequences, with corresponding safety levels of one, two and three. Different categories of structures or components should be selected with different steel, heavy industrial building structures, large span structures, high-rise or super high-rise civil building structures and heavy work system crane beams or structures are important first-class structures, should be selected with high quality steel; general industrial and civil building structures and other secondary structures, according to the nature of the work can be selected with ordinary quality steel. Temporary housing skeleton, general building within the subsidiary components such as ladders, railings, etc., are secondary to the three types of structure, can choose poor quality steel.
Loads can be divided into static and dynamic loads. Structures directly subjected to dynamic loads and structures in strong seismic zones should use high quality steel with good toughness and fatigue resistance, such as Q345 steel, and propose suitable additional guarantee items. And generally bear static load or indirect dynamic load of the role of structural components, at room temperature conditions can be used in the lower price of Q235 steel.
There are two types of connection methods for steel structures: welded and non-welded. Due to the welding process, welding deformation, welding stresses and other welding defects (such as biting flesh, porosity, cracks, slag, etc.) can occur, which can lead to the risk of cracks or brittle fractures in the structure. Therefore, welded structures should have stricter requirements on materials. For example, in terms of chemical composition, the limit content of carbon, sulphur and phosphorus must be strictly controlled for welded structures, while the carbon content can be reduced for non-welded structures.
4、The temperature and environment in which the structure is located
The structure is located in the environmental conditions, such as temperature changes and corrosive media, etc., on the mechanical properties of steel has a great impact. In the low temperature conditions of the structure components, especially welded structures and tensioned components, very easy to produce low-temperature cold brittle cold brittle fracture damage, should be selected with good resistance to low temperature brittle fracture performance of the township steel. Steel structures in corrosive media, such as steel plants in chemical enterprises, should use corrosion-resistant steel and strengthen the rust-proof treatment of exposed steel members. In addition, the steel of the open-air structure is prone to ageing, the steel of harmful media is prone to corrosion, fatigue and fracture, should also be differentiated to choose different materials.
Thin steel rolled more times, rolling compression ratio is large; thickness of steel compression ratio is small. Therefore, the thickness of steel is not only low strength, and plasticity, impact toughness and welding performance is also poor. Therefore, the thickness of the welded structure should be used for better material steel.
1. Shoddy steel is prone to folding. Folding is a variety of folding lines formed on the surface of the steel, this defect is often throughout the longitudinal direction of the product. The reason for folding is due to the pursuit of high efficiency of shoddy manufacturers, the amount of pressure under the large, resulting in ears, the next rolling on the folding, folding products will crack after bending, the strength of the steel big drop.
2, the appearance of shoddy steel often has a pockmarked surface phenomenon. The pockmarked surface is a defect caused by serious wear on the surface of the steel irregularities caused by rolling grooves. As a result of shoddy steel manufacturers to pursue profits, there are often rolling groove rolling most over the standard.
3, the surface of shoddy steel is prone to scarring. One is that the material of shoddy steel is not uniform, with many impurities. Another is the shoddy steel manufacturers guide guard equipment is rudimentary, easy to stick steel, these impurities bite people after the rolls easy to produce scars.
4, shoddy material surface is easy to produce cracks, the reason is that its billet is adobe, adobe porosity, adobe in the process of cooling due to the role of thermal stress, resulting in cracks, after rolling on cracks.
5, shoddy steel is easy to scratch, the reason is that shoddy steel manufacturers have rudimentary equipment, easy to produce burrs, scratching the surface of the steel. Deep scratches reduce the strength of the steel.
6, shoddy steel without metallic lustre, light red or similar to the colour of pig iron, for two reasons II, its billet is adobe.
7, shoddy steel rolling temperature is not standard, their steel temperature is measured by visual inspection, so that it can not be rolled according to the specified austenite area, the performance of steel naturally can not meet the standard.
8, shoddy steel crossbars thin and low, often appear to fill the phenomenon of insufficient, the reason is that manufacturers to achieve large negative tolerances, the finished product the first few passes of the press down the amount of large, iron type is small, the hole is not filled.