Steel pipes are closely related to human life and production activities. The production technology of the steel pipe industry not only develops rapidly but also innovates continuously, occupying an irreplaceable position in the steel industry. The development of steel pipe production technology began with the rise of the bicycle manufacturing industry. Steel pipes are not only used for transporting fluids and powdered solids, exchanging thermal energy, and manufacturing mechanical parts and containers, but they are also an economical type of steel. Using steel pipes to manufacture building structural frameworks, columns, and mechanical supports can reduce weight and save 20-40% of metal while enabling factory mechanized construction. Manufacturing highway bridges with steel pipes not only saves steel and simplifies construction but also significantly reduces the area requiring protective coatings, saving investment and maintenance costs. Therefore, no other type of steel can completely replace steel pipes, but steel pipes can replace some profiles and bars. Steel pipes have a significant relationship with the development of the national economy and the improvement of human living standards, far surpassing other types of steel. From everyday utensils, furniture, water supply and drainage systems, gas supply systems, ventilation and heating facilities to various agricultural machinery manufacturing, underground resource development, national defense equipment such as guns, bullets, missiles, rockets etc., all rely on steel pipes. 1. Inferior quality steel pipes are prone to folding. Folding refers to various crease lines formed on the surface of the steel pipe; this defect often runs through the entire length of the product. The cause of folding is due to inferior manufacturers pursuing high efficiency with excessive pressing amounts leading to ears forming; during subsequent rolling processes folding occurs which causes cracks when bent due to a significant drop in material strength. 2. Inferior quality steel pipes often exhibit a rough surface phenomenon known as 'ma mian'. This roughness is caused by severe wear on rolling grooves leading to irregularities on the surface of the material. Due to profit pursuits by inferior manufacturers, rolling grooves often exceed standards. 3. Inferior quality steel pipes are prone to scabs on their surfaces for two reasons: (1) The material is uneven with many impurities; (2) The equipment used by inferior manufacturers is rudimentary which makes it easy for impurities to stick during rolling processes leading to scabs. 4. Inferior materials easily develop cracks on their surfaces because their billets are made from clay which has many pores; during cooling due to thermal stress cracks form which result in visible cracks after rolling. 5. Inferior quality steel pipes are easily scratched because manufacturers use rudimentary equipment that produces burrs which scratch the surface of the material. Deep scratches reduce material strength. 6. Inferior quality steel pipes lack metallic luster appearing light red or resembling pig iron color for two reasons: (1) Their billets are made from clay; (2) The rolling temperature is not standardized as they visually estimate temperatures making it impossible to roll within specified austenite regions resulting in substandard material properties. 7. Inferior quality steel pipes have thin and low transverse ribs often showing incomplete filling due to manufacturers aiming for large negative tolerances resulting in excessive pressing amounts before final product formation leading to small iron shapes that do not fill properly. 8. The cross-section of inferior quality steel pipes appears oval because manufacturers excessively press before final product formation in order to save materials causing significant drops in strength that do not meet standard dimensions for rebar. 9. High-quality materials have uniform composition with high tonnage cold shears producing smooth and neat cut ends while inferior materials often show meat loss at cut ends resulting in uneven surfaces without metallic luster; additionally products from inferior manufacturers have fewer cut ends resulting in large ears at both ends. 10. Inferior quality steel pipe materials contain many impurities with lower density and serious dimensional deviations so without calipers one can weigh them for verification purposes; for example regarding rebar size #20 according to national standards maximum negative tolerance is set at 5%, when measuring a standard length of 9M its theoretical weight should be around 120 kg thus its minimum weight should be: 120X(1-5%)=114 kg if actual measured weight falls below this then it indicates inferior material since negative tolerance exceeds 5%. Generally speaking weighing whole batches yields better results mainly considering cumulative errors and probability issues. 11. The inner diameter dimensions of inferior quality steel pipes fluctuate greatly due to: (1) Unstable temperatures causing yin-yang surfaces; (2) Uneven composition; (3) Due to rudimentary equipment with low foundation strength causing significant bouncing during rolling processes leading to considerable variations within same week making such rebar prone to uneven stress resulting in breakage. 12. High-quality tubes have standardized trademarks and markings. 13. For large diameter threaded tubes over #16 from San Steel Pipe brand there should be a spacing greater than IM between trademarks. 14. Inferior threaded rebar often exhibits wavy longitudinal ribs; furthermore due lack of cranes at inferior manufacturers packaging tends be loose resulting in oval shapes.
