GB/T 700-2006 Grade D Double-Sided Submerged Arc Welding (DSAW) Steel Pipe
Basic Overview
GB/T 700-2006 Grade D (Q235D) Double-Sided Submerged Arc Welding (DSAW) steel pipe is the highest quality grade of carbon structural steel pipe, specifically designed for severe cold environments requiring guaranteed impact toughness at -20°C. Grade D features the tightest chemical controls among all Q235 grades and offers the best low-temperature performance, making it suitable for critical applications in arctic regions, offshore platforms, and infrastructure projects in severe cold climates [citation:3, citation:5].
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 demanding applications where maximum reliability and low-temperature performance are required .
Name Explanation
| Part | Meaning |
|---|---|
| GB/T | Chinese national standard (Guobiao/Tuijian) |
| 700-2006 | Standard number for Carbon Structural Steels (2006 edition) |
| Grade D | Highest quality grade – guarantees impact toughness of 27 J at -20°C; tightest controls on P (0.035%) and S (0.035%) |
| 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 | Q235D – Minimum yield strength 235 MPa (for thickness ≤ 16mm) |
| Quality Level | Grade D – Guaranteed impact toughness at -20°C (27 J minimum); tightest chemical controls |
| Deoxidation Method | TZ (Fully Killed Steel) – Complete deoxidation with nitrogen-binding elements for fine grain structure |
| 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 D)
| Element | Maximum % | Notes |
|---|---|---|
| Carbon (C) | 0.17% | Lowest carbon among Q235 grades; improves low-temperature toughness and weldability [citation:4, citation:6, citation:7] |
| Silicon (Si) | 0.35% | Deoxidizer [citation:4, citation:6, citation:7] |
| Manganese (Mn) | 1.40% | Provides strength [citation:4, citation:6, citation:7] |
| Phosphorus (P) | 0.035% | Tightest control among Q235 grades; critical for low-temperature performance [citation:3, citation:4, citation:6, citation:7] |
| Sulfur (S) | 0.035% | Tightest control among Q235 grades; reduces hot cracking risk and improves low-temperature toughness [citation:3, citation:4, citation:6, citation:7] |
*Note: Grade D requires fully killed steel (TZ) with sufficient grain-refining elements. When aluminum deoxidation is used, acid-soluble aluminum content shall not be less than 0.015%, or total aluminum content not less than 0.020% .*
Q235 Grade Comparison (Quality Levels)
| Grade | Impact Test Temperature | P max (%) | S max (%) | C max (%) | Deoxidation | Key Feature |
|---|---|---|---|---|---|---|
| Q235A | Not guaranteed | 0.045 | 0.050 | 0.22 | F, Z | Most economical; general structures [citation:3, citation:9] |
| Q235B | +20°C (27J min) | 0.045 | 0.045 | 0.20 | F, Z | General construction, mild climates [citation:3, citation:9] |
| Q235C | 0°C (27J min) | 0.040 | 0.040 | 0.17 | Z | Colder climates, outdoor structures [citation:3, citation:9] |
| Q235D | -20°C (27J min) | 0.035 | 0.035 | 0.17 | TZ | Severe cold environments, critical applications [citation:3, citation:9] |
*Grade D is the only Q235 grade that requires fully killed steel (TZ) and offers guaranteed impact toughness at -20°C, making it the optimal choice for low-temperature applications [citation:1, citation:5].*
Mechanical Properties (GB/T 700-2006)
| Property | Q235D Requirement | Condition |
|---|---|---|
| Yield Strength (min) | 235 MPa | Thickness ≤ 16mm |
| 225 MPa | 16mm < t ≤ 40mm | |
| 215 MPa | 40mm < t ≤ 60mm | |
| 215 MPa | 60mm < t ≤ 100mm | |
| 195 MPa | 100mm < t ≤ 150mm | |
| 185 MPa | t > 150mm | |
| Tensile Strength | 370-500 MPa | All thicknesses [citation:3, citation:7] |
| Elongation (min) | 26% | Thickness ≤ 40mm |
| 25% | 40mm < t ≤ 60mm | |
| 24% | 60mm < t ≤ 100mm | |
| 22% | 100mm < t ≤ 150mm | |
| 21% | t > 150mm | |
| Charpy Impact (Grade D) | 27 J at -20°C | Mandatory – the defining characteristic of Grade D [citation:3, citation:5] |
| Cold Bend Test (180°) | d = 1.5a | For thickness ≤ 60mm (longitudinal) |
| d = 2a | For thickness > 60-100mm (longitudinal) |
*Note: Q235D exhibits the smallest decrease in impact toughness after work hardening, making it the preferred material for cold-formed components in low-temperature environments [citation:2, citation:8].*
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 Q235D requirements (fully killed steel)
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% coverage of weld seam
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 tight composition limits per GB/T 700; mandatory for Grade D |
| 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 D | 27 J at -20°C – the defining test for Grade D [citation:3, citation:5] |
| Hydrostatic Test | Each pipe | Pressure integrity verification |
| Ultrasonic Inspection | 100% of weld seam | Continuous automatic flaw detection |
| Grain Size Verification | Required for Grade D | Ensures fine grain structure from fully killed steel practice |
| 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
Q235D DSAW pipes are specifically suitable for severe cold environments and critical applications:
| Application | Description |
|---|---|
| Arctic and Sub-Arctic Construction | Building frames, columns, trusses in regions with extreme winter temperatures (Northeast China, Siberia, Northern Europe, Canada, Alaska) |
| Offshore Platforms | Marine structures in cold water environments requiring guaranteed low-temperature toughness |
| Critical Infrastructure | Bridges, transmission towers, highway structures in severe cold climates |
| Low-Pressure Fluid Transmission | Water, gas pipelines in arctic regions |
| Wind Turbine Towers | Large-diameter support structures for wind energy projects in cold climates [citation:2, citation:8] |
| Piling Foundations | Load-bearing piles for buildings in permafrost regions |
| Heavy Machinery | Equipment requiring guaranteed toughness at -20°C |
| Pressure Vessels | Low-temperature service applications (with appropriate design code) |
GB/T 700-2006 Grade D vs. Grades A/B/C Comparison
| Feature | Grade A | Grade B | Grade C | Grade D |
|---|---|---|---|---|
| Impact Testing | Not required | 27J at +20°C | 27J at 0°C | 27J at -20°C |
| Carbon (C) max | 0.22% | 0.20% | 0.17% | 0.17% |
| Phosphorus (P) max | 0.045% | 0.045% | 0.040% | 0.035% |
| Sulfur (S) max | 0.050% | 0.045% | 0.040% | 0.035% |
| Deoxidation | F, Z | F, Z | Z | TZ (Fully Killed) |
| Typical Application | Indoor, non-critical | General construction, mild climates | Colder climates, outdoor structures | Severe cold environments, critical applications |
| Relative Cost | Lowest | Low | Medium | Highest |
*Grade D represents the highest quality level in the Q235 family, offering the best low-temperature performance and tightest chemical controls [citation:3, citation:5].*
Research & Performance Data
According to research published in the journal "Environmental Technology" (2023), Q235D exhibits superior low-temperature impact toughness compared to other Q235 grades:
Q235D material shows the smallest decrease in impact toughness after work hardening
The average impact toughness of Q235D is least affected by the cold-forming process
Q235D is the optimal choice for components requiring cold forming in low-temperature environments [citation:2, citation:8]
This makes Grade D particularly suitable for:
Cold-formed structural components in arctic regions
Components subjected to cold working during fabrication
Applications requiring reliable performance at sub-zero temperatures
Coating Options
Q235D 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 |
| IPN8710 | Water pipelines | High resistance to water corrosion |
| TPEP (3PE + Internal Epoxy) | Dual protection | Internal epoxy + external 3PE |
| Cement Mortar Lining | Water pipelines | Internal corrosion protection |
| 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) | 250 MPa yield; different impact temperature requirements |
| EN 10025 | S235J2 | 235 MPa yield, 27J at -20°C – direct European equivalent |
| JIS G3101 | SS400 | 245 MPa yield; impact requirements differ |
| DIN | ST37-3 | Historical German equivalent |
| ISO 630 | E235B | International standard equivalent |
Key Advantages
| Advantage | Description |
|---|---|
| Highest Quality Level | Best low-temperature impact toughness among Q235 grades [citation:2, citation:8] |
| Guaranteed -20°C Toughness | 27 J at -20°C ensures reliable performance in severe cold environments [citation:3, citation:5] |
| Tightest Chemical Controls | Lowest P (0.035%) and S (0.035%) limits for maximum material purity [citation:3, citation:4] |
| Fully Killed Steel (TZ) | Complete deoxidation with fine grain structure for consistent low-temperature properties |
| Excellent Weldability | Low carbon content (≤0.17%) allows welding with minimal preheat |
| Superior Cold Forming | Smallest decrease in impact toughness after work hardening, ideal for cold-formed components [citation:2, citation:8] |
| 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 |
| Proven Performance | Research-validated low-temperature impact resistance [citation:2, citation:8] |
Important Considerations
1. Grade D vs. Grades A/B/C Selection
Grade A – No impact testing – indoor or non-critical applications only
Grade B – 27 J at +20°C – mild climates, general construction
Grade C – 27 J at 0°C – colder climates, outdoor structures
Grade D – 27 J at -20°C – severe cold environments, critical applications [citation:3, citation:5]
2. Regional Applications
Q235D DSAW pipes are recommended for cold regions including:
Heilongjiang, Jilin, Liaoning (Northeast China)
Inner Mongolia, Ningxia, Gansu
Xinjiang
Siberia, Northern Europe, Canada, Alaska
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
API 5L – International line pipe standard
5. Pressure Limitations
Q235D is suitable for low to medium pressure applications (typically ≤ 1.6 MPa for GB/T 3091)
For higher pressure requirements, consider Q355 or API 5L grades
6. Temperature Limitations
Maximum recommended service temperature: 350°C (662°F)
Short-term exposure up to 450°C possible
Minimum service temperature: -20°C (with guaranteed impact toughness)
For applications below -20°C, consider low-temperature carbon steels (e.g., ASTM A333 Gr.6)
7. Coating for Corrosion Protection
For buried applications in cold regions, specify 3LPE or FBE coating
For temporary protection, varnish or anti-rust oil is sufficient
For offshore applications, consider coal tar epoxy or IPN8710
8. Third-Party Inspection
Available inspection services include SGS, BV, Lloyds, TUV
9. Weldability
Low carbon content (≤0.17%) ensures excellent weldability
Suitable for all standard welding methods (SMAW, GTAW, GMAW, SAW)
No post-weld heat treatment required for most thicknesses
Preheat: 100-150°C recommended for thicker sections
Summary
GB/T 700-2006 Grade D (Q235D) Double-Sided Submerged Arc Welded (DSAW) steel pipe is the highest quality carbon structural steel pipe, specifically designed for severe cold environments requiring guaranteed impact toughness at -20°C. With minimum yield strength of 235 MPa (34,000 psi) and tensile strength of 370-500 MPa, this material represents the premium choice for critical applications in arctic regions, offshore platforms, and demanding infrastructure projects where low-temperature performance is essential.
Key features:
Material standard: GB/T 700-2006 Q235D
Grade D guarantees 27 J impact toughness at -20°C – the defining characteristic of the highest quality level [citation:3, citation:5]
Tightest chemical controls – P ≤ 0.035%, S ≤ 0.035% – ensuring maximum material purity and low-temperature performance [citation:3, citation:4]
Fully killed steel (TZ) – complete deoxidation with fine grain structure for consistent low-temperature properties
Superior cold forming – smallest decrease in impact toughness after work hardening, validated by research [citation:2, citation:8]
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
Common applications include:
Arctic and sub-arctic construction (building frames, columns, trusses)
Offshore platforms in cold water environments
Critical infrastructure in severe cold climates
Wind turbine towers in cold regions [citation:2, citation:8]
Low-pressure fluid transmission in arctic areas
Piling foundations in permafrost regions
Cold-formed components requiring reliable low-temperature toughness [citation:2, citation:8]
When ordering, specify: GB/T 700-2006, Grade Q235D, DSAW (Double-Sided Submerged Arc Welded), Size (OD x WT), Length, End Finish (plain/beveled), Coating Requirements, and Mill Test Certificate.
Grade D is the only Q235 grade that offers guaranteed impact toughness at -20°C, making it the optimal choice for severe cold environments. For general construction in mild climates where freezing is not a concern, Grade B is sufficient and more economical. For applications requiring even higher strength with low-temperature toughness, consider Q355D (GB/T 1591) instead.
