GB/T 1591-2009 Grade B (Q345B) Double-Sided Submerged Arc Welding (DSAW) Steel Pipe
Basic Overview
GB/T 1591-2009 Grade B (Q345B) Double-Sided Submerged Arc Welding (DSAW) steel pipe is a low-alloy high-strength structural steel pipe manufactured to the Chinese national standard. The "Q345B" designation indicates a minimum yield strength of 345 MPa (50,000 psi) for thicknesses ≤ 16mm, representing a significant strength upgrade over carbon steel grades like Q235 .
Grade B is the most commonly used quality level within the Q345 series, offering a balanced combination of strength, weldability, and cost-effectiveness. Unlike Grade A, Grade B guarantees Charpy impact toughness of 27-34 J at room temperature (+20°C), making it suitable for general construction and structural applications in mild climates .
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 bridges, high-rise buildings, pressure vessels, and oil/gas transmission lines .
Name Explanation
| Part | Meaning |
|---|---|
| GB/T | Chinese national standard (Guobiao/Tuijian) |
| 1591-2009 | Standard number for Low-Alloy High-Strength Structural Steels (2009 edition; note that GB/T 1591-2018 is the newer version, which replaced Q345 with Q355) |
| Grade B | Quality grade – guarantees impact toughness of 27-34 J at +20°C; tighter controls on P (0.035-0.040%) and S (0.035-0.040%) than Grade A |
| DSAW | Double-Sided Submerged Arc Welding – both inside and outside welds are applied; the welding arc is submerged in granular flux during welding |
| Also Known As | LSAW (Longitudinal Submerged Arc Welded) – when the weld seam runs longitudinally along the pipe |
Key Specifications
| Attribute | Description |
|---|---|
| Material Standard | GB/T 1591-2008/2009 – Low-Alloy High-Strength Structural Steels (2008 edition; 2009 version is common in industry references) |
| Steel Grade | Q345B – Minimum yield strength 345 MPa (for thickness ≤ 16mm) |
| Quality Level | Grade B – Guaranteed impact toughness at +20°C (27-34J minimum); tighter chemical controls than Grade A |
| Material Type | Low-alloy high-strength structural steel (HSLA) – micro-alloyed with Nb, V, Ti |
| 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 1591 Q345B)
Based on heat (ladle) analysis per GB/T 1591 requirements :
| Element | Maximum % (or Range) | Notes |
|---|---|---|
| Carbon (C) | 0.20% | Low carbon for excellent weldability |
| Silicon (Si) | 0.50-0.55% | Deoxidizer |
| Manganese (Mn) | 1.00-1.70% | Provides strength; higher than Q235 |
| Phosphorus (P) | 0.035-0.040% | Tighter than Grade A (0.045%); improves toughness |
| Sulfur (S) | 0.035-0.040% | Tighter than Grade A (0.045%); reduces hot cracking |
| Vanadium (V) | 0.02-0.15% | Micro-alloying for grain refinement |
| Niobium (Nb) | 0.015-0.060% | Micro-alloying for strength |
| Titanium (Ti) | 0.02-0.20% | Micro-alloying for grain refinement |
| Chromium (Cr) | 0.30% max | Residual element |
| Nickel (Ni) | 0.50% max | Residual element |
| Copper (Cu) | 0.30% max | Residual element |
| Nitrogen (N) | 0.012% max | Controlled |
| Molybdenum (Mo) | 0.10% max | Residual element |
*Note: Grade B has stricter phosphorus and sulfur limits (0.035-0.040% vs. 0.045%) compared to Grade A, contributing to better weldability and impact toughness .*
Q345 Grade Comparison (Quality Levels)
| Grade | Impact Test Temperature | P max (%) | S max (%) | Al min (%) | Key Feature |
|---|---|---|---|---|---|
| Q345A | Not required | 0.045 | 0.045 | Not specified | Most economical; general structures |
| Q345B | +20°C (27-34J min) | 0.035-0.040 | 0.035-0.040 | Not specified | General construction, mild climates |
| Q345C | 0°C (34J min) | 0.030-0.035 | 0.030-0.035 | ≥0.015 | Colder climates, outdoor structures |
| Q345D | -20°C (34J min) | 0.025-0.030 | 0.025-0.030 | ≥0.015 | Severe cold environments |
| Q345E | -40°C (27J min) | 0.020-0.025 | 0.015-0.020 | ≥0.015 | Extreme cold, critical applications |
Grade B is the most common grade for general construction, offering guaranteed impact toughness at room temperature .
Mechanical Properties (GB/T 1591 Q345B)
Yield Strength (min)
| Thickness (mm) | Yield Strength (MPa) |
|---|---|
| ≤ 16 | 345 |
| 16 < t ≤ 35 | 325 |
| 35 < t ≤ 50 | 295 |
| 50 < t ≤ 100 | 275 |
| 100 < t ≤ 150 | 285 (per some sources) |
| 150 < t ≤ 200 | 275 (per some sources) |
Tensile Strength
| Thickness (mm) | Tensile Strength (MPa) |
|---|---|
| ≤ 100 | 470-630 |
| 100 < t ≤ 250 | 450-600 |
Other Properties
| Property | Requirement |
|---|---|
| Elongation (min) | 20-23% (depends on thickness) |
| Charpy Impact (Grade B) | 27-34 J at +20°C – the defining characteristic of Grade B |
| Density | 7.85 g/cm³ |
| Modulus of Elasticity | 200-206 GPa |
| Hardness | 150-200 HB typical |
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 |
| Out-of-Roundness | ≤ 1% of specified OD |
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 1591 Q345B 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% coverage of weld seam
Hydrostatic Pressure Test – Each pipe undergoes hydrostatic pressure testing
Finishing – End processing, beveling, coating, and marking according to specifications
Testing & Inspection Requirements
| Test Type | Requirement | Notes |
|---|---|---|
| Chemical Analysis | Per heat lot | Verifies composition limits per GB/T 1591 |
| 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 | 27-34 J at +20°C – the defining test for Grade B |
| Hydrostatic Test | Each pipe | Pressure integrity verification |
| Ultrasonic Inspection | 100% of weld seam | Continuous automatic flaw detection; can be performed as per ASTM E213, A388, or ASME SA388 |
| X-ray Inspection | For "T" joints | Where strip butt welds intersect spiral seams |
| Dimensional Inspection | 100% | Per standard tolerances |
| Mill Test Certificate | EN 10204 Type 3.1B | Provided with full test results |
Delivery Conditions
Q345B DSAW pipes can be supplied in various delivery conditions :
| Condition | Description | Typical Application |
|---|---|---|
| AR | As Hot Rolled only | General structural applications |
| TMCP | Thermal Mechanical Control Processing | Enhanced toughness and weldability |
| N | Normalized | Improved grain structure |
| Q+T | Quenched and Tempered | Maximum strength uniformity |
Supplementary Requirements available :
Z Direction Test (Z15, Z25, Z35) – Through-thickness properties for critical applications
Charpy V-Notch Impact Test – At specified temperatures
The Third Party Test – SGS, BV, Lloyds inspection available
Coated or Shot Blasting and Painting – Surface protection
Applications
Q345B DSAW pipes are widely used across multiple industries :
| Application | Description |
|---|---|
| High-Rise Building Construction | Support columns, steel structure frames for skyscrapers and tall buildings |
| Bridge Engineering | Main beams, structural members, bridge piers |
| Oil and Gas Transmission | Long-distance pipelines for crude oil, natural gas, and refined products |
| Water Transmission Mains | Large-diameter municipal and industrial water pipelines |
| Pressure Vessels | Tanks and vessels for non-corrosive fluids |
| Offshore Platforms | Marine structures requiring high strength |
| Piling Foundations | Load-bearing piles for buildings and structures |
| Wind Turbine Towers | Large-diameter support structures for wind energy projects |
| Industrial Process Piping | Chemical plants, power generation, steel mills |
| Heavy Machinery | Crane booms, excavator components, agricultural equipment |
GB/T 1591-2009 Grade B vs. Grade A Comparison
| Feature | Grade A (Q345A) | Grade B (Q345B) | Significance |
|---|---|---|---|
| Impact Testing | Not required | Mandatory (27-34J at +20°C) | Grade B guarantees room temperature toughness |
| Phosphorus (P) max | 0.045% | 0.035-0.040% | Grade B has tighter controls for better weldability |
| Sulfur (S) max | 0.045% | 0.035-0.040% | Grade B has tighter controls for improved toughness |
| Typical Application | Non-critical, indoor structures | General construction, mild climates | Grade B is preferred for most structural applications |
| Relative Cost | Lowest | Low | Grade B offers better value for most projects |
*The key differentiator is impact toughness – Q345B guarantees 27-34J at +20°C, while Q345A has no guaranteed impact properties .*
Q345B vs. Q235B Comparison
| Property | Q345B | Q235B | Significance |
|---|---|---|---|
| Yield Strength (min) | 345 MPa | 235 MPa | Q345 offers 47% higher strength |
| Tensile Strength (min) | 470 MPa | 370 MPa | Q345 has higher tensile strength |
| Material Type | Low-alloy HSLA | Carbon steel | Q345 has better strength-to-weight ratio |
| Micro-alloying | Nb, V, Ti added | None | Q345 has finer grain structure |
| Typical Application | Higher-load structures | General construction | Q345 for more demanding applications |
Coating Options
Q345B 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 |
| Coal Tar Epoxy | Marine environments | Heavy-duty 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 |
| Shot Blasting and Painting | Surface preparation | Improved coating adhesion |
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 Equivalents
| Standard | Equivalent Grade | Notes |
|---|---|---|
| ASTM | A572 Grade 50 | Similar yield strength (345-450 MPa), different impact requirements |
| EN 10025 | S355JR | 355 MPa yield, 27J at +20°C – direct European equivalent |
| ISO 630 | E355 | International standard equivalent |
| JIS G3106 | SM490 | Japanese standard |
| DIN | St52-3 | Historical German equivalent |
Key Advantages
| Advantage | Description |
|---|---|
| High Strength | 345 MPa yield – 47% higher than Q235, allowing reduced wall thickness or higher load capacity |
| Guaranteed Toughness | 27-34 J at +20°C ensures reliable performance at room temperature |
| Excellent Weldability | Low carbon content (≤0.20%) and controlled carbon equivalent ensure good weldability |
| Micro-alloyed | Contains Nb, V, Ti for grain refinement and improved properties |
| 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 |
| Wide Availability | Most common Q345 grade; readily available from Chinese manufacturers |
| Cost-Effective | Offers excellent value for general construction applications |
Important Considerations
1. Grade B vs. Other Quality Grades
Grade A – No impact testing – non-critical, indoor applications only
Grade B – 27-34J at +20°C – general construction, mild climates
Grade C – 34J at 0°C – colder climates, outdoor structures
Grade D – 34J at -20°C – severe cold environments
Grade E – 27J at -40°C – extreme cold, critical applications
2. Impact Testing Requirements
Grade B guarantees impact toughness of 27-34J at +20°C
For applications below 0°C, specify Grade C, D, or E instead
3. Standard Version Note
GB/T 1591-2009 has been superseded by GB/T 1591-2018, which replaced Q345 with Q355 (minimum yield strength adjusted to 355 MPa)
When ordering, specify the required version year to ensure compliance
For new projects, Q355 series is recommended
4. Product Standard
While GB/T 1591 defines the material grade, the finished DSAW pipe product is typically manufactured to product standards such as:
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 (when specified)
5. Pressure Limitations
Q345B is suitable for low to medium pressure applications
For higher pressure requirements, consult design codes for allowable stress values
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 at +20°C)
For applications below 0°C, specify Grade C (0°C) or 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
For marine environments, consider coal tar epoxy
8. Third-Party Inspection
Available inspection services include SGS, BV, Lloyds
9. Weldability
Low carbon content (≤0.20%) 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
10. Minimum Order Quantity
Typical MOQ: 1 piece
Larger orders available from manufacturers
Summary
GB/T 1591-2009 Grade B (Q345B) Double-Sided Submerged Arc Welded (DSAW) steel pipe is the most common low-alloy high-strength structural steel pipe for general construction applications. With minimum yield strength of 345 MPa (50,000 psi) – approximately 47% higher than Q235 – and guaranteed Charpy impact toughness of 27-34 J at room temperature (+20°C), this material offers an excellent balance of strength, weldability, and cost-effectiveness for a wide range of demanding applications .
Key features:
Material standard: GB/T 1591-2009 Q345B (being replaced by Q355 in GB/T 1591-2018)
Grade B guarantees 27-34 J impact toughness at +20°C – the key differentiator from Grade A
Tighter chemical controls – P ≤ 0.035-0.040%, S ≤ 0.035-0.040% – better than Grade A
Micro-alloyed with Nb, V, Ti for grain refinement and enhanced properties
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 Q345 grade for general construction
Common applications include:
High-rise building construction (support columns, steel frames)
Bridge engineering (main beams, structural members)
Oil and gas transmission pipelines
Water transmission mains and pressure vessels
Offshore platforms and piling foundations
Heavy machinery and industrial process piping
When ordering, specify: GB/T 1591-2009, Grade Q345B, DSAW (Double-Sided Submerged Arc Welded), Size (OD x WT), Length, End Finish (plain/beveled), Delivery Condition (AR/TMCP/N/Q+T), Coating Requirements, and Mill Test Certificate to EN 10204 Type 3.1B .
Grade B is the recommended choice for general construction in mild climates where guaranteed room temperature toughness is required. For applications requiring low-temperature toughness (0°C or below), specify Grade C (0°C), Grade D (-20°C), or Grade E (-40°C) instead. For new projects, consider the newer Q355 series under GB/T 1591-2018.
