Q235D is a carbon structural steel grade specified under the Chinese standard GB/T 700. It is the highest quality grade within the Q235 series. The nomenclature breaks down as follows:
Q: Denotes Yield Strength (from the Chinese word "Qu" - 屈)
235: Indicates the minimum yield strength is 235 MPa (for nominal thickness ≤ 16mm).
D: Represents the quality grade, which primarily defines impact toughness and chemical composition control.
Pipes manufactured from Q235D are typically produced according to standards like GB/T 8162 (for structural seamless pipes) or GB/T 8163 (for seamless pipes for fluid transmission). Q235D is characterized by its excellent low-temperature impact toughness, making it suitable for critical structures and applications in cold environments where brittle fracture is a concern.
Key Product Parameters & Characteristics
1. Chemical Composition (Melting Analysis)
The chemical composition is tightly controlled to ensure good weldability and low-temperature performance.
| Element | Content (%) (Standard Requirement, GB/T 700) | Typical Value (%) |
|---|---|---|
| Carbon (C) | ≤ 0.17 | 0.12 - 0.17 |
| Manganese (Mn) | ≤ 1.40 | 0.35 - 0.70 |
| Silicon (Si) | ≤ 0.35 | 0.20 - 0.35 |
| Sulfur (S) | ≤ 0.030 | 0.020 - 0.030 |
| Phosphorus (P) | ≤ 0.030 | 0.020 - 0.030 |
| De-oxidation Method | Special Killed Steel (Fully deoxidized, typically with Aluminum) |
Note: The very low Sulfur and Phosphorus content improves toughness. The steel must be specially killed, which provides a fine-grained structure essential for good impact properties.
2. Mechanical Properties
These properties define the steel's behavior under load. Tested on the base material of the pipe.
| Property | Standard Requirement (GB/T 700) | Value / Description |
|---|---|---|
| Yield Strength (ReH) | ≥ 235 MPa | ~235-275 MPa (Value decreases with increasing wall thickness) |
| Tensile Strength (Rm) | 375 - 500 MPa | ~375-460 MPa |
| Elongation at Break (A) | ≥ 25% (for thickness 40-60mm) | Typically ~26-33% |
| Impact Energy (KV2) | ≥ 27 J at -20°C (-4°F) | This is the key distinguishing feature of Q235D. Tested longitudinally on Charpy V-notch specimens at -20°C. |
3. Dimensional Specifications (GB/T 8162/8163)
Q235D pipes cover a wide range of standard sizes.
| Parameter | Common Range | Standard Specifications |
|---|---|---|
| Outer Diameter (OD) | 10 mm - 660 mm (and larger) | Governed by the pipe standard (e.g., GB/T 8162) |
| Wall Thickness (WT) | 1.0 mm - 60 mm (and thicker) | Governed by the pipe standard |
| Length | Usually 6m, 12m, or Random | Fixed Length (6m/12m) or Random Length (RLL) |
4. Technological Properties
| Property | Performance |
|---|---|
| Weldability | Excellent. Due to its low carbon equivalent, it is very easy to weld using all common methods (SMAW, GMAW, etc.). Pre-heating may be recommended for very thick sections or in very cold environments. |
| Bending/Forming | Very Good. It has high ductility and can be cold formed easily. |
| Machinability | Good. Typical machinability rating for low-carbon steel. |
Comparison Table of Q235 Grades (A, B, C, D)
This table clearly shows the progression in quality within the Q235 grade family.
| Feature | Q235A | Q235B | Q235C | Q235D |
|---|---|---|---|---|
| Impact Test | Not Required | At +20°C (68°F) | At 0°C (32°F) | At -20°C (-4°F) |
| Impact Energy (min) | - | ≥ 27 J | ≥ 27 J | ≥ 27 J |
| S (Sulfur) Max | 0.050% | 0.045% | 0.040% | 0.030% |
| P (Phosphorus) Max | 0.045% | 0.045% | 0.040% | 0.030% |
| De-oxidation | F, Z | Z | Z | Z (Special Killed) |
| Application | General structures, no impact req. | General structures at ambient temp. | Structures in slightly low temps | Critical structures in cold climates |
Legend: F = Rimmed steel, Z = Killed steel.
Equivalent Grades (Approximate)
Important: Direct equivalents are rare due to differences in national standards. Q235D is best specified directly. However, these grades serve similar mechanical and application purposes.
| Standard | Approximate Equivalent / Comparable Grade |
|---|---|
| Europe (EN 10025-2) | S235J2 (Tested at -20°C). This is a very close match in terms of mechanical properties and application. |
| USA (ASTM) | ASTM A573 Gr. 58 is a structural steel with similar strength and notch toughness requirements. For piping, the toughness level is similar to ASTM A333 Grade 1 (but A333 Gr.1 has different strength levels). ASTM A36 does not have inherent impact test requirements. |
| International (ISO) | E235C or E235D (ISO 630) - The "D" designation also indicates testing at -20°C. |
| Japan (JIS G 3101) | SS330 or SS400 (but these grades have no guaranteed impact properties, so Q235D is superior). |
Applications of Q235D Steel Pipes
Critical structural components in low-temperature environments (e.g., outdoor bridges, towers in cold regions).
Heavy-duty manufacturing equipment and frames requiring high impact resistance.
Lifting equipment and safety-critical hooks and frames.
Pressure vessels and piping for non-corrosive services where low-temperature toughness is mandated by design codes.
Offshore and shipbuilding applications (for non-primary structures where specific classification society grades are not required).
Important Note for Specification:
When procuring Q235D pipe, it is critical to specify both the material standard (GB/T 700) and the pipe product standard (e.g., GB/T 8162 for structural use). This ensures all dimensional tolerances, non-destructive testing, and marking requirements are met in addition to the chemical and mechanical properties.
