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How To Choose Good Steels How To Identify Poor Quality Steels

How to choose good steels

1、Importance of the structure about choose good steels

According to the “Uniform Standards for Structural Reliability Design of Buildings” (GB50068-2001), structures and members can be divided into three categories of importance, general and minor according to their uses, parts and severity of damage consequences, and the corresponding safety levels are 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 used for ordinary quality steel. Temporary housing skeleton, general buildings within the subsidiary components such as ladders, railings, etc., are secondary to the three types of structure, can choose poor quality  good steels.

How To Choose Good Steel How To Identify Poor Quality Steels

2、Load situation
The load can be divided into two kinds of static load and dynamic load. Structures directly subjected to dynamic loads and structures in strong seismic zones should use high-quality good steels with good toughness and fatigue resistance, such as Q345 good steels, and propose suitable additional guarantee items. And generally bear static load or indirect dynamic load effect of the structural elements, in room temperature conditions can be used in the lower price of Q235 good steels.

3、Connecting methods
There are two types of connection methods for good steels structures: welded and non-welded. As the welding process will produce welding deformation, welding stress and other welding defects (such as biting flesh, porosity, cracks, slag, etc.), there is a risk of causing cracks or brittle fractures in the structure. Therefore, the welded structure should have stricter requirements on the material. For example, in terms of chemical composition, the welded structure must strictly control the limit content of carbon, sulfur and phosphorus, while the non-welded structure of carbon content can be reduced requirements.

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 good steels has a great impact. In low-temperature conditions of the structural components, especially welded structures and tensioned components, is very susceptible to low-temperature cold brittle cold brittle fracture damage, should be selected with good resistance to low-temperature brittle fracture performance of the township steel. In the corrosive medium of the steel structure, such as the steel plant of chemical enterprises, should use corrosion-resistant steel, and strengthen the exposed steel members of the anti-rust treatment. In addition, the steel of the open-air structure is susceptible to aging, the role of harmful media good steel susceptible to corrosion, fatigue and fracture, should also be differentiated to choose different materials.

5、Steel thickness
Thin steel rolled more times, rolling compression ratio is large; thickness of good steels 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.

How to identify poor quality 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 press down is large, generating ears, the next rolling on the folding, folding products will crack after bending, the strength of steel big drop.
  2. shoddy steel often has a pockmarked appearance. The pockmarked surface is a defect caused by serious wear of the rolling groove steel surface irregularities of the bump. As the 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 the shoddy steel material is not uniform, impurities. The other is the shoddy steel manufacturers guide equipment is rudimentary, easy to stick steel, these impurities bite the rollers easy to produce scars.
  4. shoddy material surface is easy to produce cracks, the reason is that its billet is adobe, adobe porous, adobe in the process of cooling due to the role of thermal stress, resulting in cracks, after the rolling of 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 scratching reduces the strength of the steel.
  6. shoddy steel without metallic luster, light red or similar to the color of pig iron, for two reasons II, its billet is adobe.
  7. shoddy steel rolling temperature is not standard, their steel temperature is measured visually, so that it can not be rolled according to the specified austenite area, the performance of the 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 the manufacturer to achieve large negative tolerances, the finished product before the first few passes of the pressure down the amount of large, small iron type, the hole is not filled.
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