GB/T 6725-2017 Cold Forming Steel Sections Product Standard

1. Definition

GB/T 6725-2017 (General requirements of cold forming steel sections, Chinese national recommended standard) specifies general technical requirements for cold-formed steel sections produced by continuous roll cold forming from steel strip or plate. It is a product standard governing the delivered section quality, not a structural design code.

Roll forming plants in China and export markets referencing Chinese material specifications commonly align production acceptance and MTC content with GB/T 6725 unless a project specifies a different product standard (e.g., EN 10162, ASTM A1003 themes).

2. Scope

The standard covers:

• Open sections (channels, angles, Z-purlins, custom profiles) from cold roll forming
• Closed hollow sections formed from strip with longitudinal seam welding (square/rectangular tubes)
• Sections for general structural, building, and industrial uses where referenced by design codes or project specs
• Requirements linked to base metal standards for strip (carbon structural steel, low alloy, galvanized substrates per referenced GB materials)

It does not replace profile-specific standards (e.g., dedicated C-channel dimension standards) where projects cite them alongside GB/T 6725 general requirements.

3. Continuous Roll Cold Forming

GB/T 6725 explicitly addresses sections made by continuous roll cold forming from strip — the core process of roll forming lines:

1. Uncoil and level strip
2. Progressive forming through roll stands to target profile
3. In-line punching or notching if required before cutoff
4. Cut to length
5. For hollow types: high-frequency induction welding (HF welder) or other approved seam welding, then sizing and straightening

Standard sets acceptance criteria on the finished section, independent of machine brand. Process parameters (pass count, speed) remain manufacturer proprietary within quality results meeting standard limits.

4. Order Contents

Purchase orders or contracts should specify:

5. Dimensions and Tolerances

GB/T 6725 defines dimensional deviation limits for section geometry:

• Section height and width tolerances relative to nominal
• Leg length and angle deviations for asymmetric profiles
• Wall thickness tolerance considering strip thickness variation and forming thinning
• Straightness (bow, camber, twist) limits along length
• Cut length tolerance and end squareness
• Weld seam position and external flash control themes for hollow sections

Roll formers verify dimensions with profile gauges and CMM sampling per quality plan. Tighter tolerances than standard may be contractually agreed but must be measurable and documented.

6. Mechanical Properties

Mechanical requirements tie to base material grade on MTC:

• Yield strength, tensile strength, elongation per steel grade standard
• Cold forming work hardening may increase local strength at bends; product standard references base metal tests on strip or finished section as specified
• Bend tests or flare tests for hollow sections where standard requires weld ductility demonstration
• Impact properties if low-temperature grade ordered per material standard

Design engineers using GB/T 50018 (structure design) take mechanical properties from material grade but apply resistance factors in the design code — not in GB/T 6725.

7. Surface Quality

Surface acceptance includes:

• Freedom from defects affecting use: cracks, splits, rolling marks beyond limits
• Edge condition without excessive burr from cutoff or punching
• Galvanized or coated surface continuity when coated strip supplied
• Repair limitations for surface defects per standard or project agreement
• Oil, rust, or contamination limits at shipment for downstream painting or assembly

Pre-painted strip forming inherits coating defect rules from coating product standards plus forming scrape limits agreed with customer.

8. Hollow Section Weld Quality

For closed sections with longitudinal seam weld, GB/T 6725 addresses weld quality themes:

• Weld continuity and freedom from cracks, porosity, undercut beyond limits
• External flash height controlled; internal flash may be limited per agreement for fluid or fit-up applications
• HF weld heat-affected zone not compromising mechanical tests
• Nondestructive testing (NDT) themes when contract or application requires enhanced assurance
• Flattening or expansion tests demonstrating seam ductility on sampled lengths

Weld quality depends on roll former HF welder setup, strip edge preparation, and strip fit-up geometry. See ZTRFM Wiki topics on rectangular tube and welding process control.

9. Packaging and MTC

Delivery requirements include:

• Bundling with steel or PET strapping; corner protection to avoid coating damage
• Labeling: grade, batch, heat number, size, weight, manufacturer
• Material test certificate (MTC) showing chemical and mechanical results per batch
• Inspection certificate content per GB/T 6725 and referenced material standards
• Export shipments may add English MTC and third-party inspection (SGS, BV themes) if contract requires

Traceability from MTC heat number to bundle label is standard buyer audit item on structural projects.

10. Product vs Design Standard

Do not confuse GB/T 6725 with GB/T 50018 (cold-formed steel structure design):

See ZTRFM Wiki: GB/T 50018 for design code transition themes.

11. Roll Former Quality Themes

Plants certifying output to GB/T 6725 typically control:

• Incoming strip inspection against PO grade and thickness
• First-article dimensional sign-off on profile change
• In-process checks for twist and bow on long lengths
• HF welder parameter logs for hollow products
• ISO 9001 QMS backing document control (see ZTRFM Wiki: ISO 9001)

12. Inspection and Test Methods

GB/T 6725 references national test method standards for verification:

• Dimensional inspection with steel tape, calipers, and profile templates at sampling frequency per quality plan
• Tensile tests on strip or sample coupons per GB/T 228 themes for mechanical property confirmation
• Bend or flatten tests on hollow section samples demonstrating weld ductility
• Visual and gauge inspection of weld seam continuity on tube products
• Coating thickness measurement on galvanized sections when coated after forming or from coated strip
• Review of chemical composition on MTC from steel mill heat analysis

Third-party inspection agencies may witness sampling on export contracts. Inspection scope should cite GB/T 6725 clause references agreed in contract technical schedule.

13. Export and Cross-Standard Supply

Chinese roll formers exporting sections globally often produce to GB/T 6725 while customers reference other product standards:

• Engineer equivalency study maps Q345 to ASTM A572 or S355 themes where approved
• Dimensional tolerances compared between GB/T 6725 and EN 10162 for acceptance
• English MTC translation with heat number traceability required by overseas EPC contractors
• Combined container shipments label bundles per profile, grade, and standard cited on PO

Export does not automatically change factory internal standard; PO must state acceptance standard explicitly.

14. Boundaries

This page summarizes GB/T 6725-2017 themes for international buyers sourcing Chinese roll-formed sections. Numeric limits must be read from the official standard text in Chinese or authorized translation. Not structural design advice.

15. Buyer / Engineer FAQ

Is GB/T 6725 mandatory?

Recommended national standard (GB/T). Project specs or regulations may make it binding by reference.

Can we use GB/T 6725 sections with AISI S100 design?

Cross-jurisdiction design requires engineer judgment on material mapping and approval; product standard alone does not enable automatic cross-code use.

What MTC fields are required?

Grade, dimensions, chemical/mechanical results, batch ID, standard reference per GB/T 6725 and material standard.

Does GB/T 6725 cover hot-rolled sections?

No. It targets cold-formed sections from strip via roll forming or related cold processes.

How does hollow tube weld quality get verified?

Visual, dimensional, and mechanical sample tests per standard; additional NDT if contract specifies.

Is this a design code?

No. It is a product standard for manufactured sections.

What inspection frequency is typical on production runs?

Defined in manufacturer quality plan: often first-article full check, then periodic dimensional samples per batch size and profile criticality agreed with customer.

Can GB/T 6725 sections be galvanized after forming?

Yes when contract specifies post-galvanizing to relevant coating standards; dimensional growth and threading tolerances must be agreed before order.

16. Related Encyclopedia Topics

• GB/T 50018 Structure Design Standard
• Material Certificate MTC
• Rectangular Tube Roll Forming
• Standards Families Overview

17. Summary for Specifiers

GB/T 6725-2017 defines general requirements for cold-formed steel sections produced by continuous roll forming from strip: ordering information, dimensional tolerances, mechanical property linkage, surface quality, hollow-section weld quality, packaging, and MTC content. It is a product standard, not a structural design code. Pair with GB/T 50018 or applicable design specification for engineering calculations.