Description
Direct Mill Supply, No Middlemen — Q195 through Q355 carbon steel square and rectangular tubes produced and fabricated under one roof, with full material traceability from billet to finished section.
Technical Specifications
| Parameter | Value |
|---|---|
| Designation | Q235B/Q355 Carbon Steel Square/Rectangular Tube |
| Product Type | Square / Rectangular Steel Tube (SHS/RHS) |
| Steel Grade | Q195, Q215, Q235, Q345, Q355, SGCC, DX51D |
| Standard | ASTM, GB, BS, EN, DIN |
| Section Shape | Square, Rectangular (customizable) |
| Size Range | 30×30mm – 200×400mm |
| Wall Thickness | 1.5mm – 12mm |
| Length | 1 – 12m |
| Technique | ERW / Seamless |
| Surface Treatment | Black / Galvanized |
| Tolerance | ±1% |
| Ends | Plain |
| Processing Service | Welding, Punching, Cutting, Bending, Decoiling |
| Application | Indoor Structural / Architectural Applications, Steel Structure Engineering, Construction, Curtain Wall Construction, Greenhouse |
| MOQ | 1 Ton |
| Origin | Shandong, China |
Application Suitability
| Industry | Typical Applications |
|---|---|
| Steel Structure Engineering | Building frames, column-beam assemblies, platform supports, truss systems |
| Construction | Scaffolding uprights, purlin systems, bracing members |
| Greenhouse | Structural skeleton for agricultural greenhouses, roof arches, side posts |
| Curtain Wall Construction | Mullion frames, facade support grids, architectural exposed columns |
| Indoor Structural / Architectural | Mezzanine floors, partition frames, decorative structural elements |
What Negative Tolerance Actually Costs on a Carbon Steel Square Rectangular Tube Order
A wall thickness that measures 1.3mm instead of 1.5mm can void an entire structural inspection.
I watched a container of 100×100 square tubes get rejected at a Vietnamese job site because the buyer’s caliper read 0.2mm below spec on every piece. The entire lot came back to the mill for re-rolling. Overseas buyers sourcing from fragmented suppliers face this exact risk — the mill test certificate says one thing, the physical product says another, and by the time you discover it, the ship has already sailed. Material traceability gaps between the steel producer and the tube fabricator make the problem worse. You end up with no accountability and a project schedule that slips by weeks. [NEED_CITE: EN 10219 cold-formed hollow section dimensional tolerance requirements]
Matching Q235B and Q355 Sections to Structural Frame Requirements
When a structural engineer specifies Q235B for a warehouse frame or Q355 for a heavier industrial platform, the carbon steel square rectangular tube must deliver consistent yield behavior across every heat. Our vertically integrated line — from raw steel production through ERW and seamless tube forming — ensures the chemical composition and mechanical properties on the mill test certificate match what arrives on site. For greenhouse structures in humid tropical climates, the galvanized finish option provides corrosion protection without additional field coating, while black finish tubes serve fabrication shops that need clean weld surfaces for on-site assembly.
Engineering Compliance Across International Standards
Structural projects in the EU require EN 10219 or EN 10210 compliance for hollow sections, while North American projects reference ASTM A500. Our carbon steel square rectangular tubes are produced to meet GB, EN, ASTM, BS, and DIN standards depending on the destination market. [NEED_CITE: steel grade equivalence mapping between GB Q-series and EN S-series standards] The ±1% dimensional tolerance holds across the full 30×30mm to 200×400mm range, which matters when shop drawings call for precise connection node dimensions. For curtain wall applications where exposed tubes carry architectural finishes, surface quality and consistent corner radii become critical acceptance criteria.
Understanding Grade Selection and Production Methods
The Q-series grading system follows Chinese GB standards where the number indicates minimum yield strength in MPa — Q235 delivers approximately 235 MPa yield, while Q355 steps up for heavier structural demands. This parallels the EN system where S235JR and S355JR serve similar structural roles. [NEED_CITE: Q235 to S235JR grade equivalence in structural applications] ERW tubes suit general structural and architectural applications where cost efficiency matters, while seamless tubes handle higher-pressure or critical-load scenarios. For wall thickness, the 1.5mm to 12mm range covers everything from light greenhouse framing to heavy industrial columns. Galvanized finish adds zinc coating for outdoor or corrosive environments; black finish remains the default for fabrication shops that will weld, paint, or powder-coat after assembly.
Hidden Costs of Sourcing Without Verified Certification
Pricing that looks attractive on paper often hides corners cut elsewhere — off-grade steel substituted without disclosure, wall thickness pushed to the lower edge of negative tolerance, or mill test certificates generated without actual batch testing. The consequences are not abstract: customs holds when documentation does not match shipment, structural inspection failures that halt construction, and re-fabrication costs that dwarf the original savings. [NEED_CITE: material certification requirements for structural steel imports in EU and Middle East markets] A carbon steel square rectangular tube from a verified integrated producer eliminates these variables because the same facility controls material selection, forming, testing, and documentation.
Why Buyers Trust Our Integrated Production Line
Our mill produces the raw steel, and our fabrication division forms it into finished tubes — one facility, one quality system, one set of documentation. This eliminates the traceability gap that plagues orders routed through trading companies who source tubes from one mill, steel from another, and coatings from a third. We maintain over 2,800 stock specifications covering the full Q195–Q355 grade range and 30×30mm to 200×400mm size spectrum, so project buyers do not wait for custom rolling on standard sections. Every order ships with a mill test certificate tied to the specific heat number, and third-party inspection through SGS or BV is available before loading. For oversized or heavy-wall sections, our loading team handles specialized lashing and container optimization — the same crew that has shipped structural steel to the Middle East, Southeast Asia, and Africa knows how to prevent transit damage on galvanized surfaces and precision-cut ends.
Documentation & Compliance
- Mill test certificate documenting Q235B/Q355 chemical composition and mechanical properties per GB/T 6728, with heat number traceability
- CE certificate for EN 10219 cold-formed structural hollow sections when shipping to EU markets
- ISO 9001 quality management system certification covering the full production and inspection process
- SGS or BV third-party inspection report available upon request, covering dimensional checks, coating thickness, and weld seam quality
- Material traceability report linking each bundle to its original heat number and rolling batch
- Complete customs documentation including packing list, bill of lading, and certificate of origin
Packaging, Loading & Delivery
- Q235B and Q355 tubes bundled with steel strip ties and edge protectors to maintain ±1% dimensional integrity during ocean transit
- Galvanized square and rectangular tubes wrapped with waterproof paper and plastic sleeves to preserve zinc coating surface quality
- Custom cut-to-length service available across the 1–12m range, with bundles labeled by size, grade, and heat number for site identification
- Container loading optimized by our team for 30×30mm through 200×400mm sections, maximizing payload while preventing nested damage
- FOB, CIF, and DDP terms available with door-to-door logistics coordination for project sites across the Middle East, Southeast Asia, and Africa
Start Your Procurement With Verified Specifications
Share your project drawings, required steel grades (Q195 through Q355), section sizes, surface treatment preference, and destination port — we respond within 24 hours with a detailed quotation including material cost, fabrication, coating, packing, and shipping, along with a shop drawing timeline and production schedule.
Request a Quote With Mill Test Certificate
Send your specifications and quantity to receive pricing, material documentation, and delivery timeline.
Mobile/WhatsApp/WeChat: +86 188 5315 7756
Email: admin@xjystructure.com
Frequently Asked Questions
Q: How do Q235B and Q355 grades compare to international standards like S235JR or A36?
A: Q235B corresponds closely to S235JR in the EN system and A36 in ASTM, offering similar yield strength for general structural applications. Q355 aligns with S355JR for higher-load structural requirements. We provide grade equivalence documentation with every order to support engineering approval and customs clearance.
Q: Can I request third-party inspection before shipment?
A: Yes. SGS and BV third-party inspections are available for every order. The inspection covers dimensional verification, wall thickness measurement, coating thickness for galvanized tubes, and weld seam quality for ERW products. The inspection report ships with your documentation package.
Q: What is the minimum order quantity for custom sizes outside the standard range?
A: Our standard MOQ is 1 Ton. For non-standard sizes outside the 30×30mm to 200×400mm range, we evaluate based on production feasibility and can discuss custom rolling requirements. Contact us with your specific dimensions for a tailored assessment.
Q: How do I choose between galvanized and black finish for my project?
A: Galvanized finish is recommended for outdoor applications, greenhouse structures, or corrosive environments where long-term corrosion resistance is needed without field painting. Black finish suits indoor structural applications, fabrication shops that will weld or paint on-site, and projects where surface coating will be applied after assembly.
Q: How does theoretical weight invoicing work with the ±1% tolerance?
A: We invoice by theoretical weight calculated from the nominal dimensions and standard steel density. The ±1% tolerance applies to actual physical dimensions. This is standard industry practice for carbon steel square and rectangular tubes and ensures transparent, predictable pricing regardless of minor production variations.
Get Your Carbon Steel Square Rectangular Tube Quote Today
Submit your section sizes, steel grade, quantity, and destination port to receive a complete quotation with mill test certificate and production timeline.
Mobile/WhatsApp/WeChat: +86 188 5315 7756
Email: admin@xjystructure.com
