Q235B is a very common and widely used carbon structural steel grade in China, specified under the standard GB/T 700-2006. The "Q" stands for the yield point (Yield Strength), "235" indicates the minimum yield strength of 235 MPa, and "B" denotes the quality grade, which specifies the impact test temperature (room temperature 20°C) and chemical composition.
Pipes made from Q235B are typically manufactured as Welded Steel Pipes (e.g., ERW, LSAW, SSAW pipes). They are rarely produced as seamless pipes, as seamless pipes usually require higher-grade materials like 20# or 16Mn.
Key Characteristics:
Good Plasticity and Toughness: Suitable for forming and welding.
Moderate Strength: Ideal for structural applications without extreme load requirements.
Excellent Weldability: Can be easily welded using all standard methods.
Cost-Effective: One of the most economical steel choices available.
1. Chemical Composition (GB/T 700-2006)
The following table outlines the maximum percentage of chemical elements by weight:
| Element | Carbon (C) | Silicon (Si) | Manganese (Mn) | Phosphorus (P) | Sulfur (S) |
|---|---|---|---|---|---|
| Content % (Max) | 0.20% | 0.35% | 1.40% | 0.045% | 0.045% |
| Remark | Impurity (Harmful) | Impurity (Harmful) |
Note: The contents of Arsenic (As) and Residual elements may also be limited.
2. Mechanical Properties
The mechanical properties are the core parameters defining this grade.
| Property | Yield Strength (Reh) | Tensile Strength (Rm) | Elongation (A) | Impact Energy (KV) @ 20°C |
|---|---|---|---|---|
| Value (Min) | ≥ 235 MPa | 370 - 500 MPa | ≥ 26% | ≥ 27 Joules |
| Test Standard | GB/T 228.1 | GB/T 228.1 | GB/T 228.1 | GB/T 229 |
*Note: The tensile strength range (370-500 MPa) is a typical requirement. The actual value must be greater than 370 MPa but less than 500 MPa to ensure proper ductility.*
3. Common Specifications and Dimensions (GB/T 3091-2015, GB/T 21835-2008)
Q235B welded pipes are available in a vast range of sizes.
| Parameter | Typical Range | Common Standards |
|---|---|---|
| Outer Diameter (OD) | ½" (21.3mm) to 96" (~2400mm+) | Commonly from 20mm to 2000mm |
| Wall Thickness (WT) | 1.0mm to 25.4mm (1") and above | Varies with diameter and process |
| Length | Random (5m-12m) or Fixed Length (6m, 12m) | As per customer requirement |
| Manufacturing Process | ERW (Electric Resistance Welded), LSAW (Longitudinal Submerged Arc Welded), SSAW (Spiral Submerged Arc Welded) |
Example Size Table (ERW Pipe):
| Nominal Bore (NPS) | Outer Diameter (OD) | Wall Thickness Schedule (WT) |
|---|---|---|
| ½ inch | 21.3 mm | 2.0 mm, 2.6 mm, 3.2 mm |
| 1 inch | 33.4 mm | 2.6 mm, 3.2 mm, 4.0 mm |
| 2 inch | 60.3 mm | 2.6 mm, 3.2 mm, 4.0 mm |
| 4 inch | 114.3 mm | 3.2 mm, 4.0 mm, 5.0 mm, 6.3 mm |
| 8 inch | 219.1 mm | 4.0 mm, 5.0 mm, 6.3 mm, 8.0 mm |
| 12 inch | 323.9 mm | 5.0 mm, 6.3 mm, 8.0 mm, 10.0 mm |
4. Standards and Applications
| Application Sector | Common Chinese Standards | Equivalent Int'l Standards (Similar) | Primary Use |
|---|---|---|---|
| Low-Pressure Fluid Transport | GB/T 3091 | ASTM A53 Gr. B (Note 1) | Water, gas, air, heating systems |
| General Structures | GB/T 6728 (Square/Rectangular) | ASTM A500 Gr. B/C (Note 2) | Building frames, support structures, fences |
| Fencing, Handrails | GB/T 3091, GB/T 6728 | - | Guardrails, scaffolding (non-critical parts) |
Important Notes on Equivalents:
Note 1: While often compared, Q235B is not fully equivalent to ASTM A53 Gr. B. A53 Gr. B has a higher minimum yield strength (≥ 240 MPa vs. 235 MPa) and different chemical requirements (e.g., higher Cu content). They are, however, used in similar applications.
Note 2: ASTM A500 Gr. B is a standard for structural tubing and has different mechanical property requirements than Q235B. They are not direct equivalents but serve similar structural purposes.
5. Comparison with Other Common Grades
| Grade | Standard | Min Yield Strength | Key Difference |
|---|---|---|---|
| Q235B | GB/T 700 | 235 MPa | Standard carbon steel, good for welding. |
| Q355B | GB/T 1591 | 355 MPa | Higher strength low-alloy steel. Replaces old Q345B. |
| 20# | GB/T 699 | 245 MPa | "High-Quality" Carbon Steel, better for machining/seamless pipes. |
| ASTM A53 Gr. B | ASTM A53 | 240 MPa | Int'l standard for fluid transport, slightly different chemistry. |
Summary
Q235B steel pipe is a versatile, economical, and readily available product ideal for low-to-medium pressure fluid transportation and general structural applications where high strength is not the primary concern. Its excellent weldability and formability make it a fundamental material in construction and infrastructure projects, primarily in China and many other countries. Always confirm the specific standard (e.g., GB/T 3091) when ordering to ensure the dimensional and testing requirements are met for your project.
