GB/T 700-2006 Grade B Double-Sided Submerged Arc Welding (DSAW) Steel Pipe
Basic Overview
GB/T 700-2006 Grade B (Q235B) Double-Sided Submerged Arc Welding (DSAW) steel pipe is the most common carbon structural steel pipe grade in China, widely used for general construction, structural supports, and low-pressure fluid transmission. The "Grade B" designation guarantees a minimum impact toughness of 27 J at room temperature (+20°C) , distinguishing it from Grade A which has no impact requirements.
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, typically in separate processes, 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.
Name Explanation
| Part | Meaning |
|---|---|
| GB/T | Chinese national standard (Guobiao/Tuijian) |
| 700-2006 | Standard number for Carbon Structural Steels (2006 edition) |
| Grade B | Quality grade – guarantees impact toughness of 27 J at +20°C ; tighter controls on S (0.045%) than Grade A (0.050%) |
| 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 (current version; GB/T 700-2019 is newer) |
| Steel Grade | Q235B – Minimum yield strength 235 MPa (for thickness ≤ 16mm) |
| Quality Level | Grade B – Guaranteed impact toughness at room temperature (27 J at +20°C); tighter S limit (0.045%) |
| 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 B)
| Element | Maximum % | Notes |
|---|---|---|
| Carbon (C) | 0.20% | Lower than Grade A (0.22%); better weldability |
| Silicon (Si) | 0.35% | Deoxidizer |
| Manganese (Mn) | 1.40% | Provides strength |
| Phosphorus (P) | 0.045% | Same as Grade A; controlled for weldability |
| Sulfur (S) | 0.045% | Tighter than Grade A (0.050%); improves toughness |
Note: Grade B has lower carbon content (0.20% vs. 0.22%) and tighter sulfur control (0.045% vs. 0.050%) than Grade A, contributing to better weldability and impact toughness.
Q235 Grade Comparison (Quality Levels)
| Grade | Impact Test Temperature | P max (%) | S max (%) | Key Feature |
|---|---|---|---|---|
| Q235A | Not guaranteed | 0.045 | 0.050 | Most economical; general structures |
| Q235B | +20°C (27J min) | 0.045 | 0.045 | General construction, mild climates |
| Q235C | 0°C (27J min) | 0.040 | 0.040 | Colder climates, outdoor structures |
| Q235D | -20°C (27J min) | 0.035 | 0.035 | Severe cold environments |
Grade B is the most common grade for general construction, offering guaranteed impact toughness at room temperature.
Mechanical Properties (GB/T 700-2006)
| Property | Q235B 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 | 370-500 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 B) | 27 J at +20°C | Mandatory – the defining characteristic of Grade B |
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 Q235B 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
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 B | 27 J at +20°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 |
Common Applications
| Application | Description |
|---|---|
| General Structural Engineering | Building frames, columns, trusses for industrial plants, stadiums |
| Low-Pressure Fluid Transmission | Water, gas, air, and heating systems |
| Piling Foundations | Load-bearing piles for buildings and structures |
| Infrastructure Projects | Tunnel supports, retaining walls, highway structures |
| Mechanical Manufacturing | Equipment frames, machinery bases |
| Construction Applications | Scaffolding, handrails, fencing |
GB/T 700-2006 Grade B vs. Grade A Comparison
| Feature | Grade A (Q235A) | Grade B (Q235B) | Significance |
|---|---|---|---|
| Impact Testing | Not required | Mandatory (27J at +20°C) | Grade B guarantees room temperature toughness |
| Carbon (C) max | 0.22% | 0.20% | Grade B has lower carbon for better weldability |
| Sulfur (S) max | 0.050% | 0.045% | Grade B has tighter sulfur control |
| Typical Application | Indoor, non-critical structures | General construction, mild climates | Grade B is the standard choice for most projects |
Coating Options
Q235B 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 A53 | Gr. B | Similar yield strength (240 MPa vs. 235 MPa) |
| EN 10025 | S235JR | 235 MPa yield, 27J at +20°C |
| JIS G3101 | SS400 | 245 MPa yield |
| DIN | ST37-2 | Historical German equivalent |
Important Note on Equivalents: While often compared to ASTM A53 Gr. B, Q235B is not fully equivalent. A53 Gr. B has a higher minimum yield strength (≥ 240 MPa vs. 235 MPa) and different chemical requirements. They are, however, used in similar applications.
Key Advantages
| Advantage | Description |
|---|---|
| Most Common Grade | Q235B is the standard choice for general construction in China |
| Guaranteed Toughness | 27 J at +20°C ensures reliable performance at room temperature |
| Excellent Weldability | Low carbon content (≤0.20%) allows welding with minimal preheat |
| Good Formability | Suitable for cold bending, flanging, and shaping without cracking |
| 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 | Economical choice for general structural applications |
| Wide Availability | Commonly stocked by Chinese manufacturers |
Important Considerations
1. Grade B vs. Grade A Selection
Grade B is the standard choice for most construction applications – guarantees room temperature impact toughness
Grade A is only for non-critical, indoor applications where impact testing is not required
The small cost premium for Grade B is justified by guaranteed toughness
2. Grade B vs. Grade C/D
Grade B – 27 J at +20°C – suitable for indoor or warm climate applications
Grade C – 27 J at 0°C – for temperate climate outdoor structures
Grade D – 27 J at -20°C – for cold climates and demanding applications
For applications in cold regions, specify Grade C or D instead
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
Q235B 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
No guaranteed low-temperature toughness below 0°C – for sub-zero applications, specify Grade C (0°C) or D (-20°C)
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 (≤0.20%) ensures excellent weldability
Preheat temperature: 100-150°C recommended for thicker sections
No post-weld heat treatment required for most thicknesses
Suitable for all standard welding methods (SMAW, GTAW, GMAW, SAW)
Summary
GB/T 700-2006 Grade B (Q235B) Double-Sided Submerged Arc Welded (DSAW) steel pipe is the most common carbon structural steel pipe for general construction and low-pressure applications in China. With minimum yield strength of 235 MPa (34,000 psi) , tensile strength of 370-500 MPa, and guaranteed Charpy impact toughness of 27 J at room temperature (+20°C) , this material is the standard choice for a wide range of applications where guaranteed room temperature toughness is required.
Key features:
Material standard: GB/T 700-2006 Q235B
Grade B guarantees 27 J impact toughness at +20°C – the key difference from Grade A
Lower carbon content (0.20% vs. 0.22%) and tighter sulfur control (0.045% vs. 0.050%) than Grade A
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
Most common grade for general construction in China
Common applications include:
General structural engineering (building frames, columns, trusses)
Low-pressure fluid transmission (water, gas, air, heating systems)
Piling foundations for buildings and structures
Infrastructure projects (tunnels, retaining walls)
Construction applications (scaffolding, handrails)
When ordering, specify: GB/T 700-2006, Grade Q235B, 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, specify Grade C (0°C) or Grade D (-20°C) instead.
