Carbon steel square tubes are a common type of metal profile, named for their square or rectangular cross-section. They can be classified into square tubes and rectangular tubes based on whether the side lengths of the cross-section are equal. They are mainly made of carbon steel.

Carbon steel, also known as carbon steel, is an iron-carbon alloy with a carbon content of 0.0218% to 2.11%, and it usually also contains small amounts of silicon, manganese, sulfur and phosphorus. Generally, the higher the carbon content in carbon steel, the greater its hardness and strength, but its plasticity is relatively low.

Production mode

The main methods for producing carbon steel square tubes are seamless and welded.

Welded square tubes: They are formed by rolling hot-rolled or cold-rolled steel plates and made by high-frequency welding (ERW) or submerged arc welding (SAW). The weld seams need to be treated to ensure strength. Main process: Uncoiling and leveling → horizontal cutting and head welding → roller forming → high-frequency heating, extrusion and welding → deburring → water cooling and shaping → flaw detection → cutting and sizing → oiling for rust prevention → packaging and storage

Cold-rolled/cold-drawn square tubes: These are formed by cold working round tubes to make their surfaces smoother and achieve higher dimensional accuracy, but they are more costly. It is suitable for occasions with higher requirements for size and surface quality.

Material properties

Composition: Mainly composed of iron and carbon, with small amounts of manganese, silicon, sulfur, phosphorus and other elements.
Strength: The strength of carbon steel is closely related to its carbon content. The higher the carbon content, the higher the hardness of carbon steel, but its plasticity and toughness may decrease.
Corrosion resistance: Carbon steel has relatively poor corrosion resistance and requires surface treatments such as galvanizing and painting to prevent rust.
Processability: Easy to weld, cut and bend, suitable for various processing requirements.

Common specifications: Side lengths range from 20mm, 25mm, 30mm, 40mm... Ranging to 600 mm; The wall thickness ranges from 1.5 mm, 2 mm... Up to 20mm is common.
External dimension deviation: ± 0.60mm... ± 5.00mm, depending on the cross-sectional dimensions and wall thickness.
Wall thickness deviation: + 0.15mm / -0.10mm (thin wall); + 0.20mm / -0.15mm (thick wall), etc.

Common pipe types: A513/A500 are the most common types of steel pipes. High strength, good weldability and low cost.
A513 mechanical tube: Outer diameter less than 2 inches or wall thickness less than 11ga.
Mechanical properties: Tensile strength: 602Mpa Yield strength: 512Mpa

A500 structural pipe: Outer diameter 2 inches or more, wall thickness greater than 11ga.
Mechanical properties:
Grade A: Tensile strength: 310Mpa Yield strength: 269Mpa
Grade B: Tensile strength: 400Mpa Yield strength: 317Mpa
Grade C: Tensile strength: 427Mpa Yield strength: 345Mpa
Grade D: Tensile strength: 400Mpa Yield strength: 250Mpa

Welding and quality control
Pre-welding preparation: Remove oil stains, rust and oxide scale from the surface of the base material; Local preheating (>100 °C) should be carried out when necessary to prevent cold cracking.

Bevel processing: V-shaped bevels are commonly used, with a total bevel Angle of 55° - 72° and a blunt edge of 0.5-1 mm, ensuring the formation and fusion of the weld bead.

Welding process:

Base welding: Manual tungsten inert gas welding (TIG)

Filling cover surface: Flux-cored wire CO₂ shielded welding (MIG/MAG), short-circuit transition, molten pool width 5-12 mm, welding speed 6-15 cm/min.

Non-destructive testing: ultrasonic testing (UT), radiographic testing (RT), magnetic particle testing (MT), and hydrostatic testing of weld seams, etc., to ensure that the weld seams are free of defects such as pores, slag inclusions, and incomplete fusion.

Quality acceptance: The appearance should be free of cracks, undercut and severe deformation. The dimensional tolerance and straightness meet the standard requirements; The mechanical properties meet the requirements stipulated in the contract or standards.

Application field

Building structures: scaffolding, curtain wall keels, stair railings, steel structure frames, etc.
Mechanical manufacturing: lathes, machine tool beds, frames, conveying equipment, etc.
Transportation: Automobile chassis, trailer frames, boat deck supports, etc.
Furniture and decoration: office desks and chairs, shelves, screens, bathroom hangers, etc.
Energy and Chemical Engineering: Solar brackets, pipeline transportation, chemical equipment support, etc.