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Help CenterSquare Tube & Square Pipe，Q235B、Q355B、6MnDG、Q355NH
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1. Product Definition
Square tube, full name Cold-Formed Square Hollow Section (SHS). Rectangular Hollow Section (RHS) is its matching product. It is square hollow structural steel formed by cold bending, welding or extrusion of steel strip and seamless round pipe. Marking format: Side Length × Side Length × Wall Thickness (e.g. 50×50×3). Standard fixed lengths are 6m, 9m and 12m, widely used as load-bearing profiles in construction, machinery and decoration industries.
2. Product Classification (3 Categories)
2.1 By Production Process
High-Frequency Longitudinal Welded Square Tube (Mainstream, 90% Market Share)Steel strip cold forming + high-frequency resistance welding with polished and removed internal & external weld burrs. Featuring high dimensional accuracy and cost performance. Applicable for small & medium sizes (10×10~400×400mm), widely used in general steel structures, storage racks and greenhouses.
Seamless Square Tube (Weld-Free, For High Pressure & Heavy Load)Manufactured by hot rolling or cold drawing extrusion of seamless round pipes, divided into hot-rolled seamless and precision cold-drawn seamless types. Superior pressure resistance, torsion resistance and impact resistance without welding defects. Suitable for hydraulic equipment, engineering machinery, heavy-duty supports and fluid pipelines.
Submerged Arc Welding Large-Diameter Heavy-Wall Square TubeFor side length ≥400mm and wall thickness ≥12mm heavy-duty tubes, applied in factory columns, bridges, pipe piles and large steel structure bases.
2.2 By Material
Carbon Steel Square Tube (General Civil & Industrial Use)
Q195: Soft material, easy to bend, for furniture, decoration and simple brackets with low strength.
Q235B (Most Common Grade): Yield strength ≥235MPa, for normal-temperature buildings, workshops, guardrails and storage racks.
Q355B (Former Q345B): High-strength low-alloy steel, yield strength ≥355MPa, for long-span steel structures, curtain wall keels, vehicle chassis and heavy machinery.
Low-temperature Special Grades: Q355D/Q355E, for outdoor projects in cold northern regions with sub-zero temperatures.
20#, 45#: Precision cold-drawn seamless square tubes, for machining, shaft sleeves and hydraulic components.
Alloy Steel Square Tube (Special Working Conditions)
40Cr, 42CrMo, high strength and wear resistance, for engineering machinery and mold supports.
2.3 By Surface Treatment
Black Square Tube (Uncoated)No anti-corrosion treatment after welding with black oxide surface. Lowest cost, for indoor use; customers shall apply paint or spray coating for rust prevention after purchase.
Hot-Dip Galvanized Square Tube (Outdoor Preferred)Immersed in molten zinc at 450℃, zinc coating weight 200~600g/㎡ (≥70μm). Firm zinc-iron alloy layer, no red rust after 48-hour salt spray test. Used for greenhouses, guardrails, photovoltaic brackets and outdoor steel structures, service life 10~20 years.
Pre-Galvanized Square TubeFormed from pre-galvanized steel strip, thin zinc coating 40~80g/㎡. For indoor decoration and simple racks, not suitable for long-term rain exposure.
Deep Processing Surface TreatmentSandblasting, electrostatic powder coating, fluorocarbon coating, pickling & passivation, for high-end decoration and anti-corrosion equipment.
3. Specification Parameters
3.1 Size Range
Side length: Mini 10×10mm ~ Large 600×600mm
Wall thickness: 0.6mm ~ 32mm; regular stock 2~12mm, customized heavy wall 14~32mm
Standard length: 5.8m/6m/9m/12m; custom cut length 2~12m available
3.2 Dimensional Tolerance (GB/T 6728)
Side length tolerance: ±1.0mm
Wall thickness tolerance: ±5%~±10% (precision cold-drawn tube ±0.15mm)
Straightness: ≤2mm per meter, end face perpendicularity ≤1°
3.3 Theoretical Weight Calculation Formula
Weight per meter (kg/m) = (Side Length mm − Wall Thickness mm) × 4 × Wall Thickness mm × 0.00785Example: 50×50×3 square tube = (50−3)×4×3×0.00785 ≈ 4.42kg/m
4. Executive Standards
Domestic Standard: GB/T 6728-2017 Cold-Formed Structural Steel Sections (Core standard for structural square tubes), GB/T 3091 Welded Steel Pipes
International Standards: EN10219 (EU), ASTM A500 (USA), JIS G3466 (Japan). CE & ISO9001 certified products available.
5. Core Product Performance
Mechanical Properties (Q235B)Yield strength ≥235MPa, tensile strength 375~500MPa, elongation after fracture ≥21%. Support bending, welding, drilling and cutting.
Structural Advantages
Uniform stress distribution on square section, better bending & torsion resistance than round tubes; right-angle positioning for easy assembly and welding.
Light self-weight with large section modulus; over 60% lighter than solid steel under equal load, reducing overall project cost.
Flat four sides, convenient for painting, galvanizing and cladding decorative panels.
WeldabilityCarbon square tubes compatible with MIG/MAG welding, manual arc welding and TIG welding. Seamless tubes free of welding defects for high-pressure load-bearing parts.
6. Production Process
6.1 High-Frequency Welded Square Tube Process
Steel strip uncoiling → leveling → cold forming → high-frequency welding → internal & external weld deburring → fixed-length cutting → straightening → flaw detection → surface treatment (black / galvanized) → packing & warehousing
6.2 Cold-Drawn Seamless Square Tube Process
Hot rolled round pipe → pickling & phosphating → multi-pass cold drawing → annealing & straightening → precision polishing → non-destructive testing → finished product
7. Inspection & Acceptance Standard
Appearance: No cracks, peeling, pores, weld beads or dents on surface; uniform galvanized coating without missing plating or peeling.
Non-destructive Testing: Eddy current / ultrasonic flaw detection, Grade II qualified welds without slag inclusion or incomplete penetration.
Physical & Chemical Test: Mechanical property (yield, tensile strength) and chemical composition sampling inspection per batch; zinc coating thickness & adhesion test for galvanized tubes.
Dimension Inspection: Calipers & square rulers to recheck side length, wall thickness and straightness.
8. Main Application Fields
Steel Construction: Factory columns, purlins, curtain wall keels, photovoltaic brackets, steel canopies, pipe piles
Agricultural Facilities: Greenhouse frames, breeding fences, agricultural machinery frames
Municipal Engineering: Road guardrails, overpass handrails, billboard supports, subway brackets
Machinery Manufacturing: Equipment frames, machine tool bases, forklift racks, engineering machinery arms
Warehousing & Logistics: Storage racks, stereo garages, turnover equipment frames
Decoration & Furniture: Door & window frames, landscape modeling, metal furniture
Special Industries: Fluid transportation, hydraulic equipment, anti-corrosion supports for chemical industry
9. Product Advantages & Disadvantages
Advantages
Regular cross-section for easy installation, splicing and drilling, improving construction efficiency.
Lightweight and high strength with good load capacity, greatly reducing overall structural load.
Diversified anti-corrosion solutions for indoor, outdoor and mild corrosive environments.
Full range of specifications, customizable non-standard size, wall thickness and length.
Multiple connection methods: welding, bolting and riveting, convenient for modification and maintenance.
Disadvantages
Thin-wall square tubes have weak local extrusion resistance; heavy-load scenarios require thicker walls.
Uncoated black tubes are prone to rust; galvanizing or spraying is mandatory for outdoor use.
Seamless square tubes cost higher than welded ones; welded tubes are preferred for budget-limited projects.
10. Packing Standard
Standard Bundle Packing (For Truck Logistics)Bound by galvanized steel wire/steel strip, each bundle 1–3 tons with binding spacing 1.2–1.5m. Plastic end caps installed on both pipe ends to prevent collision scratch and deformation during transportation. Metal tags hung on each bundle marked with specification, material, weight per meter, batch number and quantity.
Small-size / Precision Cold-Drawn Square TubeEach tube fully wrapped by thin plastic film, covered with woven bag outside and packed into small bundles to avoid surface friction scratches. Precision tubes fixed on wooden pallets additionally to prevent bending deformation.
Export & High-Standard PackingEntire tubes wrapped with waterproof stretch film, reinforced on solid wooden pallets and locked with steel strips. Waterproof plastic plugs block pipe ends. Chinese & English labels attached on pallets, moisture-proof and rust-proof for sea transportation.
Short Cut PiecesLoaded into iron bins or wooden cases separately with layered isolation to avoid edge collision damage.
11. Delivery Cycle
Hot-Dip Galvanization for Stock Tube BlanksGalvanizing processing cycle 3–5 working days, delivery immediately after processing completion.
Custom Non-Standard Products (Special side length, wall thickness, fixed cut length, Q355B/Q355D, seamless square tube, powder coating / fluorocarbon deep processing)
Custom welded square tube: 7–10 working days
Cold-drawn seamless square tube: 10–15 working days
Extra surface processing (powder coating, fluorocarbon coating): add 3–5 working days on base production cycle
Mass Orders (Single batch ≥500 tons)Production schedule negotiated mutually, regular cycle 10–20 days; partial shipment available.
Special NoticeDelivery cycle will be postponed with prior customer notice under force majeure such as rain & snow weather, environmental production control and raw material shortage. Rush order service available with negotiated extra lead time and cost.
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