Inferior steel tubes are prone to folding.
Folding refers to various creases formed on the surface of steel tubes, often running longitudinally through the entire product. This defect arises due to inferior manufacturers' pursuit of high efficiency, resulting in excessive reduction ratios that create fins, which subsequently fold during the next rolling process. Bent products with folding tend to crack, significantly reducing the steel's strength.
The surface of inferior steel tubes often exhibits pitting.
Pitting is an irregular, uneven surface defect caused by severe wear of the rolling grooves. Inferior steel tube manufacturers often exceed the rolling groove's standard usage limits to maximize profits.
Inferior steel tubes are prone to scarring on their surfaces.
This can be attributed to two reasons: (1) Uneven material composition and high impurity content in inferior steel tubes. (2) Simple guiding and defending equipment in inferior factories, leading to steel sticking and scarring after impurities embed into the rollers.
Cracks easily appear on the surface of inferior materials because their billets are earthy, containing numerous air pores. During cooling, these pores are subject to thermal stress, causing cracks that persist through the rolling process.
Inferior steel tubes are susceptible to scratching due to rudimentary equipment in inferior factories, which easily produces burrs that scratch the steel surface. Deep scratches reduce the steel's strength.
Inferior steel tubes lack metallic luster, appearing pinkish or similar to pig iron. This is due to two factors: earthy billets and inaccurate rolling temperatures. Since steel temperatures are visually estimated, they cannot be controlled within the specified austenitic range, leading to subpar steel properties.
Inferior steel tubes have thin and low transverse ribs, often appearing underfilled. This is because manufacturers aim for large negative tolerances, resulting in excessive reductions in the initial rolling stages, smaller iron dies, and underfilled pass shapes.
The cross-section of inferior steel tubes appears elliptical. Manufacturers save material by applying excessive reductions in the last two rolling stages before finishing, significantly reducing the strength of the reinforcing steel bars and violating dimensional standards.
High-quality steel has a uniform composition, processed by high-tonnage cold shears, resulting in smooth and even cut ends. In contrast, inferior materials often exhibit uneven, pitted cut ends due to poor material quality, lacking metallic luster. Furthermore, inferior factories produce fewer cut ends, leading to large fins at the head and tail.
Inferior steel tubes contain more impurities, have lower density, and significant dimensional deviations. Without a vernier caliper, they can be verified by weighing. For example, a 20mm diameter reinforcing steel bar with a maximum negative tolerance of 5% and a standard length of 9M should weigh at least 114kg (120kg x (1-5%)). Any single bar weighing less than 114kg indicates inferior steel due to excessive negative tolerances. Weighing an entire batch is more accurate, considering cumulative errors and probability theory.
The inner diameter of inferior steel tubes fluctuates significantly due to unstable steel temperatures (causing hot and cold spots), uneven material composition, and rudimentary equipment with weak foundations, leading to significant mill bounce. This results in varying bar diameters within the same week, causing uneven stress distribution and potential fractures.
High-quality tubes feature standardized trademarks and printing.
For steel tubes with diameters over 16mm, the spacing between two trademarks should exceed 1M.
The longitudinal ribs of inferior reinforcing steel bars often appear wavy.
Inferior steel tube factories often have loose packing due to the absence of overhead cranes, resulting in oval-shaped side profiles.
