GB/T 700-2006 Grade C Double-Sided Submerged Arc Welding (DSAW) Steel Pipe
Basic Overview
GB/T 700-2006 Grade C (Q235C) Double-Sided Submerged Arc Welding (DSAW) steel pipe is a carbon structural steel pipe with guaranteed low-temperature impact toughness at 0°C. This makes it suitable for outdoor structures in colder climates where Grade B's room-temperature impact guarantee is insufficient .
The DSAW (also known as LSAW - Longitudinal Submerged Arc Welding) process is a high-integrity welding method where both inside and outside welds are applied, forming a single high-quality weld nugget. This process is particularly well-suited for large-diameter, thick-wall pipes used in structural, piling, and low-pressure fluid applications in regions with freezing temperatures .
Name Explanation
| Part | Meaning |
|---|---|
| GB/T | Chinese national standard (Guobiao/Tuijian) |
| 700-2006 | Standard number for Carbon Structural Steels (2006 edition) |
| Grade C | Quality grade – guarantees impact toughness of 27 J at 0°C; tighter controls on P (0.040%) and S (0.040%) than Grade B |
| DSAW | Double-Sided Submerged Arc Welding – both inside and outside welds are applied; the welding arc is submerged in granular flux during welding |
Key Specifications
| Attribute | Description |
|---|---|
| Material Standard | GB/T 700-2006 – Carbon Structural Steels |
| Steel Grade | Q235C – Minimum yield strength 235 MPa (for thickness ≤ 16mm) |
| Quality Level | Grade C – Guaranteed impact toughness at 0°C (27 J minimum) |
| Manufacturing Process | DSAW (Double-Sided Submerged Arc Welding) / LSAW (Longitudinal Submerged Arc Welding) |
| Forming Methods | JCOE (progressive J-C-O forming) or UOE (U-ing, O-ing, Expanding) |
| Size Range | 219 mm to 1829 mm (8" to 72") OD – typical for DSAW |
| Wall Thickness | 5 mm to 40 mm (up to 50 mm for piling applications) |
| Length | 6 m to 12.3 m standard; up to 18 m available |
Chemical Composition (GB/T 700-2006 Grade C)
| Element | Maximum % | Notes |
|---|---|---|
| Carbon (C) | 0.17-0.18% | Lower than Grade A (0.22%) and Grade B (0.20%); improves low-temperature toughness |
| Silicon (Si) | 0.35% | Deoxidizer |
| Manganese (Mn) | 1.40% | Provides strength |
| Phosphorus (P) | 0.040% | Tighter than Grade B (0.045%); improves low-temperature performance |
| Sulfur (S) | 0.040% | Tighter than Grade B (0.045%); reduces hot cracking risk |
*Note: Grade C has significantly lower carbon content (≤0.17-0.18%) and tighter phosphorus/sulfur limits compared to lower grades, contributing to better low-temperature impact toughness .*
Q235 Grade Comparison (Quality Levels)
| Grade | Impact Test Temperature | P max (%) | S max (%) | C max (%) | Key Feature |
|---|---|---|---|---|---|
| Q235A | Not guaranteed | 0.045 | 0.050 | 0.22 | Most economical; general structures |
| Q235B | +20°C (27J min) | 0.045 | 0.045 | 0.20 | General construction, mild climates |
| Q235C | 0°C (27J min) | 0.040 | 0.040 | 0.17 | Colder climates, outdoor structures |
| Q235D | -20°C (27J min) | 0.035 | 0.035 | 0.17 | Severe cold environments |
Grade C offers guaranteed impact toughness at 0°C, making it suitable for outdoor structures in regions where temperatures drop to freezing .
Mechanical Properties (GB/T 700-2006)
| Property | Q235C Requirement | Condition |
|---|---|---|
| Yield Strength (min) | 235 MPa | Thickness ≤ 16mm |
| 225 MPa | 16mm < t ≤ 40mm | |
| 215 MPa | 40mm < t ≤ 60mm | |
| 205 MPa | 60mm < t ≤ 100mm | |
| 195 MPa | 100mm < t ≤ 150mm | |
| 185 MPa | t > 150mm | |
| Tensile Strength | 375-500 MPa | All thicknesses |
| Elongation (min) | 26% | Thickness ≤ 40mm |
| 25% | 40mm < t ≤ 60mm | |
| 24% | 60mm < t ≤ 100mm | |
| 23% | 100mm < t ≤ 150mm | |
| 22% | t > 150mm | |
| Charpy Impact (Grade C) | 27 J at 0°C | Mandatory – the defining characteristic of Grade C |
Dimensional Specifications
| Parameter | Range / Tolerance |
|---|---|
| Outside Diameter | 219 mm to 1829 mm (8" to 72") – typical DSAW range |
| Wall Thickness | 5 mm to 40 mm (up to 50 mm for piling) |
| Diameter Tolerance | ±0.75% of specified OD (typical) |
| Wall Thickness Tolerance | +15% / -12.5% of nominal (typical) |
| Length | 6 m to 12.3 m standard; up to 18 m available |
DSAW/LSAW Manufacturing Process
Double-Sided Submerged Arc Welding (DSAW), also known as Longitudinal Submerged Arc Welding (LSAW), is a high-efficiency process used primarily for large-diameter steel pipes :
Process Description
Welding arc is submerged in granular flux during welding, protecting the weld from atmospheric contamination
Both inside and outside welds are required, usually accomplished in separate processes
The separate welds consume a portion of the other, resulting in a single high-quality weld nugget
Forming Methods
| Method | Description | Typical Diameters |
|---|---|---|
| JCOE | Progressive J-C-O forming steps (J-shape → C-shape → O-shape), expanded after welding | 406-1626 mm (16"-64") |
| UOE | Plate pressed into U-shape, then O-shape, expanded after welding | 508-1118 mm (20"-44") |
Process Steps
Plate Selection – Steel plates meeting GB/T 700 Q235C requirements
Edge Preparation – Edges are beveled to create a V-shaped groove
Forming – Steel plates are formed into cylindrical shapes using JCOE or UOE presses
Tack Welding – Formed plates are tack-welded to maintain shape
Double Submerged Arc Welding – Pipe seam welded on both interior and exterior surfaces
Cold Expansion – Pipe may be expanded up to 1.5% to achieve final OD dimension
Ultrasonic Inspection – Seam inspected by continuous ultrasonic automatic flaw detection; ensures 100% of weld seam coverage
Hydrostatic Pressure Test – Each pipe undergoes hydrostatic pressure testing
Finishing – End processing, beveling, coating, and marking
Testing & Inspection Requirements
| Test Type | Requirement | Notes |
|---|---|---|
| Chemical Analysis | Per heat lot | Verifies composition limits per GB/T 700 |
| Tensile Test | Per lot | Verifies yield and tensile strength |
| Flattening Test | Required | Checks ductility and weld integrity |
| Bend Test | Required | Verifies weld integrity |
| Charpy Impact Test | Mandatory for Grade C | 27 J at 0°C |
| Hydrostatic Test | Each pipe | Pressure integrity verification |
| Ultrasonic Inspection | 100% of weld seam | Continuous automatic flaw detection |
| X-ray Inspection | For "T" joints | Where strip butt welds intersect spiral seams |
| Dimensional Inspection | 100% | Per standard tolerances |
| Mill Test Certificate | Provided | With full test results |
Applications
Q235C DSAW pipes are specifically suitable for regions and applications where temperatures drop to freezing :
| Application | Description |
|---|---|
| Cold Climate Construction | Building frames, columns, trusses in regions with freezing temperatures (Northeast China, Inner Mongolia, Xinjiang, Gansu, Ningxia, Heilongjiang) |
| Outdoor Structures | Bridges, transmission towers, highway structures in temperate to cold climates |
| Low-Pressure Fluid Transmission | Water, gas pipelines in regions with winter freezing |
| Piling Foundations | Load-bearing piles for buildings in areas with 0°C winter temperatures |
| Infrastructure Projects | Tunnel supports, retaining walls, port facilities in colder regions |
| Mechanical Manufacturing | Equipment requiring guaranteed toughness at 0°C |
GB/T 700-2006 Grade C vs. Grade B/D Comparison
| Feature | Grade B (Q235B) | Grade C (Q235C) | Grade D (Q235D) |
|---|---|---|---|
| Impact Testing | 27J at +20°C | 27J at 0°C | 27J at -20°C |
| Carbon (C) max | 0.20% | 0.17% | 0.17% |
| Phosphorus (P) max | 0.045% | 0.040% | 0.035% |
| Sulfur (S) max | 0.045% | 0.040% | 0.035% |
| Typical Application | General construction, mild climates | Colder climates, outdoor structures | Severe cold environments |
Grade C is the optimal choice for outdoor structures in temperate climates with freezing winter temperatures, offering better low-temperature toughness than Grade B without the higher cost of Grade D .
Coating Options
Q235C DSAW pipe can be supplied with various coatings for corrosion protection:
| Coating Type | Best For | Key Features |
|---|---|---|
| 3LPE (3-layer polyethylene) | Buried pipelines | Most common; excellent corrosion protection, high impact resistance |
| FBE (Fusion Bonded Epoxy) | Oil & gas pipelines | Strong adhesion, chemical resistance |
| Epoxy Powder Coating | Corrosive environments | Good protection, smooth surface |
| Varnish / Anti-rust oil | Temporary protection | Short-term corrosion protection during transport |
| Bitumen Coating | Buried service | Cost-effective protection |
| Black Oil Coating | Standard mill finish | Temporary protection |
End Finishes
Available end treatments include:
Plain ends (PE) – standard for most applications
Beveled ends (BE) – for field welding (30° bevel angle)
Threaded ends – for mechanical connections
International Approximations
| Standard | Equivalent Grade | Notes |
|---|---|---|
| ASTM | A36 | Similar strength; different impact test temperature |
| EN 10025 | S235J0 | 235 MPa yield, 27J at 0°C |
| JIS G3101 | SS400 | 245 MPa yield; impact requirements differ |
| DIN | ST37-3 | Historical German equivalent |
Key Advantages
| Advantage | Description |
|---|---|
| Guaranteed 0°C Toughness | 27 J at 0°C ensures reliable performance in freezing temperatures |
| Lower Carbon Content | ≤0.17-0.18% carbon improves low-temperature toughness and weldability |
| Tighter Chemistry Controls | P ≤ 0.040%, S ≤ 0.040% – stricter than Grade B, improving overall quality |
| Excellent Weldability | Low carbon content allows welding with minimal preheat |
| Large Diameter Capability | DSAW produces pipes from 8" to 72" OD – ideal for large-diameter applications |
| High Weld Integrity | Double-sided welding creates a single high-quality weld nugget with 100% UT inspection |
| Cost-Effective | More economical than Grade D while offering better low-temperature performance than Grade B |
Important Considerations
1. Grade C vs. Grade B/D Selection
Grade B – 27 J at +20°C – suitable for indoor or warm climate applications
Grade C – 27 J at 0°C – optimal for outdoor structures in temperate climates with freezing winters
Grade D – 27 J at -20°C – for severe cold environments and demanding applications
Select Grade C when the minimum service temperature is around 0°C but does not require -20°C capability
2. Regional Applications
Q235C DSAW pipes are widely used in cold regions of China including:
Jilin, Liaoning, Heilongjiang (Northeast China)
Inner Mongolia, Ningxia, Gansu
Xinjiang
3. Standard Version Note
GB/T 700-2006 has been superseded by GB/T 700-2019
When ordering, specify the required version year to ensure compliance
Most manufacturers still produce to the 2006 standard specifications
4. Product Standard
While GB/T 700 defines the material grade, the finished DSAW pipe product is typically manufactured to product standards such as:
GB/T 3091 – Welded steel pipes for low-pressure fluid transmission
GB/T 9711 – Petroleum and natural gas industries line pipe
SY/T 5037 – Spiral submerged arc welded pipe for low-pressure fluid service
5. Pressure Limitations
Q235C is suitable for low to medium pressure applications (typically ≤ 1.6 MPa for GB/T 3091)
For higher pressure requirements, consider Q355 grades
6. Temperature Limitations
Maximum recommended service temperature: 350°C (662°F)
Short-term exposure up to 450°C possible
Minimum service temperature: 0°C (with guaranteed impact toughness)
For applications below 0°C, specify Grade D (-20°C) instead
7. Coating for Corrosion Protection
For buried applications, specify 3LPE or FBE coating
For temporary protection, varnish or anti-rust oil is sufficient
8. Third-Party Inspection
Available inspection services include SGS, BV, Lloyds
9. Weldability
Low carbon content (≤0.17-0.18%) ensures excellent weldability
Suitable for all standard welding methods (SMAW, GTAW, GMAW, SAW)
No post-weld heat treatment required for most thicknesses
Summary
GB/T 700-2006 Grade C (Q235C) Double-Sided Submerged Arc Welded (DSAW) steel pipe is a carbon structural steel pipe specifically designed for applications requiring guaranteed impact toughness at 0°C. With minimum yield strength of 235 MPa (34,000 psi) and tensile strength of 375-500 MPa, this material is the optimal choice for outdoor structures in temperate climates with freezing winter temperatures.
Key features:
Material standard: GB/T 700-2006 Q235C
Grade C guarantees 27 J impact toughness at 0°C – the key differentiator from Grade B
Lower carbon content (≤0.17-0.18%) and tighter phosphorus/sulfur controls (0.040% max) than Grade B
DSAW manufacturing produces pipes from 8" to 72" OD with wall thickness up to 40 mm
Double-sided welding creates a single high-quality weld nugget
JCOE or UOE forming methods ensure precise dimensional accuracy
100% ultrasonic inspection of weld seam ensures quality
Excellent weldability and good formability
Widely used in cold regions (Northeast China, Inner Mongolia, Xinjiang, etc.)
Common applications include:
Cold climate construction (building frames, columns, trusses)
Outdoor structures in freezing temperatures (bridges, transmission towers)
Low-pressure fluid transmission in regions with winter freezing
Piling foundations and infrastructure projects
Mechanical equipment requiring guaranteed 0°C toughness
When ordering, specify: GB/T 700-2006, Grade Q235C, DSAW (Double-Sided Submerged Arc Welded), Size (OD x WT), Length, End Finish (plain/beveled), Coating Requirements, and Mill Test Certificate.
For applications requiring guaranteed low-temperature toughness at -20°C, specify Grade D instead. For general construction in mild climates where freezing is not a concern, Grade B is sufficient and more economical .
