Black square tube, also known as carbon steel square tube, is a hollow steel tube with a square or rectangular cross-section. It is named because it has a black metal surface without surface treatment (such as galvanizing or painting). Black square tubes are widely used in building structures, machinery manufacturing, steel structure engineering, automobile manufacturing and other fields, and are favored by the market for their excellent strength, durability and cost-effectiveness.

1. Production process of black square tubes
The production of black square tubes is mainly divided into two ways: seamless square tube manufacturing and welded square tube manufacturing.
1. Seamless square tube manufacturing
Seamless square tubes are formed by a series of processing and deformation of seamless round tubes. The main production process is as follows:
1. Hot-rolled or cold-drawn seamless steel tubes 2. Cutting and shaping 3. Heating 4. Perforation (if necessary) 5. Three-roll sizing or drawing 6. Cooling 7. Straightening 8. Cutting and forming 9. Inspection and storage.
The characteristics of seamless square tubes are that there are no welds, higher pressure bearing capacity and better mechanical properties, and they are often used in engineering structures with higher strength requirements.
2. Welded square tube manufacturing
Welded square tubes are made of hot-rolled or cold-rolled steel coils (strip steel) after processing and bending. The main production process is as follows:
1. Steel plate or steel strip coil 2. Opening and straightening 3. Cutting 4. Forming (cold bending or hot bending) 5. High-frequency welding 6. Weld seam trimming 7. Straightening 8. Cutting and forming 9. Inspection and storage.
Welded square tubes have low production costs and high dimensional accuracy. They are suitable for general structural parts, steel structure engineering and decoration industries.

2. Common materials of black square tubes
The main material of black square tubes is carbon steel, which can be divided into low carbon steel, medium carbon steel and high carbon steel according to different carbon content and uses. The main carbon steel materials used in the market include:
– Q195 (yield strength 195 MPa): good plasticity, suitable for low-strength structures and architectural decoration.
– Q215 (yield strength 215 MPa): higher strength than Q195, suitable for building frame structures, mechanical accessories, etc.
– Q235 (yield strength 235 MPa): the most widely used carbon steel, commonly used in construction, bridges, machinery manufacturing, etc.
– Q345 (formerly 16Mn) (yield strength 345 MPa): a low-alloy high-strength steel with higher strength than Q235, suitable for projects with high load-bearing capacity.
– 20#, 45# and other carbon structural steels: mainly used for mechanical structures and high-pressure pipelines that require good mechanical properties.
According to different national standards, some international steel grades are compared as follows:
– ASTM A500 (American standard): common grades include Grade A, Grade B, Grade C, etc.
– JIS G3466 (Japanese standard): STKR400, STKR490, etc.
– EN 10219 (European standard): S235JR, S355J2H, etc.

3. Implementation standards of black square tubes
The production and testing of black square tubes must comply with national and industry standards. Common standards are as follows:
1. GB/T 6728 (cold-bent hollow steel for structures): Applicable to structural square tubes such as buildings and machinery.
2. GB/T 3094 (cold-drawn seamless square tube): Mainly applicable to precision machining and the automotive industry.
3. GB/T 13793 (straight seam electric welded steel pipe): Specifies the production requirements for welded square tubes.
4. ASTM A500 (American standard): Applicable to square tubes for building structures and machinery manufacturing.
5. EN 10219 (European standard): non-alloy and fine-grained structural steel cold-bent hollow steel.

4. Common specifications and dimensions of black square tubes
The specifications of black square tubes are expressed in side length × wall thickness (mm), such as “50×50×3” means a square tube with a side length of 50mm and a wall thickness of 3mm. Common specifications are as follows:
1. Common sizes of square tubes
– Small size (10×10mm – 50×50mm): mainly used for furniture, decoration, mechanical parts, etc.
– Medium size (60×60mm – 150×150mm): commonly used in building frames, brackets, steel structures.
– Large size (160×160mm – 500×500mm): used for bridges, supporting structures, industrial plants, etc.
2. Common sizes of rectangular tubes
– Common aspect ratios: 30×20mm, 50×25mm, 100×50mm, 200×100mm, etc.
– Application: used for building beams and columns, mechanical components, steel structure supports, etc.
3. Wall thickness range
– Thin-walled tubes (0.5-2mm): used for decoration, furniture, and light structures.
– Medium-thick walled tubes (2-10mm): suitable for building structures and supporting frames.
– Thick-walled tubes (10-30mm or more): used for heavy machinery, bridges, pressure pipes, etc.

5. Main uses of black square tubes
Black square tubes are widely used in many industries due to their good strength, corrosion resistance and cost advantages:
1. Construction engineering
– High-rise building frame structures, factory steel structures, bridge supports, scaffolding, etc.
2. Machinery manufacturing
– Mechanical parts, agricultural machinery, automobile chassis, engineering machinery structural parts, etc.
3. Steel structure
– Carports, canopies, billboard brackets, large-span structures, etc.
4. Decoration and furniture
– Metal furniture, guardrails, door and window frames, display racks, etc.
5. Municipal and transportation
– Highway guardrails, traffic signal poles, tunnel brackets, parking sheds, etc.

As a common structural material, black square tubes have become an important product in the industrial and construction industries due to their high strength, low cost and wide application. Black square tubes of different materials, specifications and production methods are suitable for different engineering needs. When purchasing black square tubes, you need to choose the appropriate steel grade, size and standard according to the actual use to ensure the quality of the project and its service life.