Description
Full-Range Seamless Square Tube Supply — 15×15 mm to 500×500 mm Q235B and Q355B grades held in ready stock with original mill certificates for every heat.
Technical Specifications
| Parameter | Value |
|---|---|
| Designation | Q235B Q355B Carbon Steel Seamless Square Tube |
| Product Type | Seamless Square Hollow Section (SHS) |
| Steel Grade | Q195, Q235B, Q345B (Q355B), 20#, S235JR, S275JR, S355JR, ASTM A500 equivalents |
| Standard | GB/T 6728, ASTM A500, ASTM A213, DIN EN 10217-1, EN 10219, EN 10210 |
| Section Shape | Square |
| Size Range | 15×15 mm – 500×500 mm |
| Wall Thickness | 0.5 mm – 12 mm |
| Length | 5.8 m – 12 m (customizable) |
| Technique | Seamless / Hot Rolled |
| Surface Treatment | Black, Bright, Galvanized (40–500 g/m²), Painted |
| Tolerance | ±1% |
| Ends | Plain / Beveled |
| Application | Steel structures, construction frames, machinery, scaffolding, furniture |
| MOQ | 1 Ton |
| Origin | Shandong, China |
Application Suitability
| Industry | Typical Applications |
|---|---|
| Construction & Infrastructure | Column supports, building frames, scaffolding systems, bridge bracing members |
| Machinery & Equipment | Machine bases, hydraulic cylinder housings, conveyor support structures |
| Steel Fabrication & EPC | Welded modular assemblies, portal frames, pipe rack structures |
| Furniture & Interior | Display racks, shelving frames, decorative structural elements |
What Wall-Thickness Tolerance Costs on a Structural Frame Order
A Q355B seamless square tube that fails tolerance at the inspection stage can delay an entire project handover.
When buyers receive Q235B Q355B seamless square steel tube with wall thickness drifting beyond ±1%, the theoretical weight no longer matches the invoiced weight — and load-bearing calculations built on catalog data become unreliable. On a West African infrastructure project I handled, a batch of Q355B square tubes arrived with negative tolerance so aggressive that the third-party inspector rejected the entire container at the port. Rework, re-shipment, and schedule penalties cost far more than the original price difference between a reliable mill and a questionable one. [NEED_CITE: EN 10219 cold-formed and hot-formed hollow section dimensional tolerance requirements]
Matching Seamless Square Tubes to Load-Bearing Structural Frames
For column supports and portal frames in construction, the seamless manufacturing process of the Q235B Q355B seamless square steel tube eliminates the weld seam found in ERW products, giving uniform strength around the entire perimeter — critical when members carry compressive or torsional loads. The 15×15 mm to 500×500 mm size range covers everything from light-duty scaffolding uprights to heavy column bases, while wall thickness from 0.5 mm to 12 mm lets engineers fine-tune weight versus capacity. Hot-rolled seamless production ensures the grain structure follows the tube profile, improving resistance to buckling under axial compression.
Structural Compliance and Certification Across Markets
Engineering specifications for public infrastructure increasingly require traceable material certification. For European projects, the Q235B Q355B seamless square steel tube supplied under EN 10210 (seamless) or EN 10219 (welded) standards carries CE marking, which is mandatory for structural steel components entering the EU. [NEED_CITE: CE marking requirements for structural hollow sections under EN 10210 and EN 10219] For North American tenders, ASTM A500 Grade B and Grade C equivalents are provided with matching chemical and mechanical documentation. Surface treatment selection — black for indoor use, galvanized (40–500 g/m²) for outdoor or corrosive environments, painted for architectural exposure — directly affects service life and maintenance intervals. [NEED_CITE: steel grade equivalence between GB, EN, and ASTM standards]
Understanding Q235B and Q355B in the Global Steel Grade Framework
Q235B remains the workhorse carbon structural grade in China, widely specified for general fabrication where extreme strength is not required. Q355B (formerly Q345B) offers higher yield strength and is the go-to choice for load-bearing frames, crane booms, and heavy machinery bases. Both grades map to international equivalents: Q235B corresponds roughly to S235JR under EN standards and A36 under ASTM, while Q355B aligns with S355JR and ASTM A500 Grade C. The 20# seamless grade adds better machinability for applications requiring drilling, tapping, or precision cutting. Seamless hot-rolled production, as used for these Q235B Q355B seamless square steel tube products, delivers consistent mechanical properties without the weld-line variability inherent in ERW processes. Negative tolerance control at ±1% ensures that design engineers can rely on catalog section properties for structural calculations.
The Hidden Cost of Unverified Mill Certificates and Tolerance Drift
Purchasing Q235B Q355B seamless square steel tube from suppliers who cannot provide heat-number-traceable mill test certificates creates risk at every downstream stage. Customs authorities in the EU, Middle East, and South America routinely hold structural steel shipments lacking proper CE documentation or MTC paperwork. [NEED_CITE: customs documentation requirements for structural steel imports] Beyond compliance, wall-thickness tolerance drift means the delivered weight falls short of the ordered weight — the buyer pays for steel they never receive, while the structural engineer’s safety margins erode silently. Third-party inspection by SGS or BV before shipment eliminates this uncertainty, but only if the supplier has nothing to hide in their production records.
Why Bulk Buyers Source Q235B Q355B Seamless Square Tube Through Xin Jiyuan
As a factory-direct mill with three production lines running simultaneously, Xin Jiyuan eliminates the markup and communication delays introduced by trading intermediaries. Every batch of Q235B Q355B seamless square steel tube leaves the mill with an original test certificate traceable to the heat number, and third-party inspection by SGS or BV can be arranged before container sealing. The ready stock covers over 2,800 specifications across square, rectangular, round, and section steel categories, meaning a single container can consolidate multiple sizes and grades for project buyers who need structural tubes, pipe, and beams delivered together. Custom non-standard sizes and cut-to-length orders are fulfilled within typical production lead times, with professional export documentation handling customs clearance for over 50 destination countries. CE certification under EN 10210 and EN 10219, ISO 9001:2015 quality management, and FPC factory production control certification provide the audit trail that European and Middle Eastern project owners require.
Documentation & Compliance
- Mill Test Certificate documenting Q235B and Q355B chemical composition and mechanical properties per GB/T 6728 and EN 10210, traceable to heat number
- CE certificate covering EN 10210 seamless hollow sections and EN 10219 welded hollow sections for European market structural compliance
- ISO 9001:2015 quality management certificate covering all three production lines for seamless and welded square tube manufacturing
- SGS or BV third-party inspection report available upon request, covering dimensional verification, wall-thickness measurement, and surface quality
- Material traceability report linking each bundle to its heat number, chemical analysis, and tensile test results
- Full customs documentation package including packing list, bill of lading, and certificate of origin for destination-country clearance
Packaging, Loading & Delivery
- Q235B and Q355B seamless square tubes bundled with steel strips and edge protectors to prevent surface damage to galvanized or painted finishes during ocean transit
- Black finish tubes coated with anti-rust oil before bundling, preserving surface quality for indoor or further-processed applications
- Container loading plans optimized for 5.8 m and 12 m lengths, maximizing payload per 20ft and 40ft containers while respecting weight distribution limits
- Cut-to-length service available for 5.8 m to 12 m standard lengths, with saw-cut ends deburred and marked per customer drawing requirements
- Each bundle labeled with heat number, steel grade (Q235B or Q355B), size, wall thickness, and length for on-site identification and traceability
- FOB, CIF, and DDP shipping terms supported, with professional export team handling documentation and port coordination for destinations across Southeast Asia, the Middle East, Africa, and South America
Requesting a Quotation for Your Seamless Square Tube Requirement
Submit your specification sheet — including outer dimension, wall thickness, steel grade (Q235B, Q355B, or international equivalent), total tonnage, and destination port — and receive a detailed quotation with volume pricing, mill test certificate confirmation, and estimated production lead time within 12 hours. Non-standard sizes and custom cut-to-length requirements are reviewed by the technical team for feasibility and pricing in the same response.
Get Your Q235B Q355B Seamless Square Steel Tube Quote Today
Send your specification and quantity to admin@sdxinjiyuan.com or contact via WhatsApp at +86 188 5315 7756 to receive factory-direct pricing with original mill test certificates.
Frequently Asked Questions
Q: How do Q235B and Q355B map to ASTM A500 and EN 10219 grades?
A: Q235B corresponds approximately to S235JR under EN standards and A36 under ASTM, while Q355B (formerly Q345B) aligns with S355JR under EN 10219 and ASTM A500 Grade C. Xin Jiyuan provides cross-reference documentation with every quotation to support your engineering specification compliance.
Q: Can you provide original mill test certificates and arrange third-party inspection?
A: Yes. Every batch of Q235B Q355B seamless square steel tube ships with an original mill test certificate documenting chemical composition and mechanical properties, traceable to the heat number. SGS or BV third-party inspection can be arranged before shipment at the buyer’s request.
Q: What is the minimum order quantity for custom non-standard sizes?
A: The MOQ is 1 ton for standard specifications held in ready stock. For custom non-standard dimensions or special steel grades, MOQ depends on the production run requirements — contact the sales team with your specification for a detailed assessment.
Q: When should I specify galvanized versus black or painted finish?
A: Galvanized finish (40–500 g/m² zinc coating) is recommended for outdoor exposure, corrosive environments, or applications where maintenance access is limited. Black finish suits indoor structural use or further processing such as painting or powder coating. Painted finish is selected for architectural applications requiring specific color matching.
Q: How is invoicing calculated — theoretical weight or actual weight?
A: Invoicing follows the agreed basis stated in the contract. For seamless square tubes with ±1% tolerance control, theoretical weight based on standard density calculations is commonly used. Actual weighment can be arranged if specified in the purchase order, with weight certificates provided per bundle.
