Q255 Carbon Steel: Structural Carbon Steel for General Engineering and Construction Applications
Q255 carbon steel, designated under the Chinese GB/T 700 standard (equivalent to ASTM A36/A283 Grade C), is a low-to-medium carbon structural steel widely used in general engineering, construction, and manufacturing. With a minimum yield strength of 255 MPa, it offers a balanced combination of weldability, machinability, and cost-effectiveness. This article explores its chemical composition, mechanical properties, manufacturing processes, application scenarios, and key performance advantages.
1. Chemical Composition (GB/T 700 Standard)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | ≤ 0.22% | Primary strengthening element; low content ensures good weldability |
| Manganese (Mn) | 0.30 – 0.65% | Enhances strength and hardness; improves deoxidation during smelting |
| Silicon (Si) | ≤ 0.30% | Deoxidizer; improves strength without sacrificing ductility |
| Sulfur (S) | ≤ 0.045% | Impurity element; controlled to prevent hot cracking during rolling |
| Phosphorus (P) | ≤ 0.045% | Impurity element; limited to maintain toughness and weldability |
| Chromium (Cr) | ≤ 0.30% | Residual element; minimal impact on properties at this level |
| Nickel (Ni) | ≤ 0.30% | Residual element; negligible effect on mechanical properties |
| Copper (Cu) | ≤ 0.30% | Residual element; may slightly improve atmospheric corrosion resistance |
2. Mechanical Properties (Room Temperature)
- Yield Strength (σs): ≥ 255 MPa (minimum guaranteed value for structural integrity)
- Tensile Strength (σb): 410 – 550 MPa (typical range for load-bearing applications)
- Elongation (δ): ≥ 23% (50mm gauge length; ensures formability for bending and shaping)
- Impact Toughness (Akv): ≥ 27 J at 20°C (standard Charpy V-notch test)
- Hardness (HB): ≤ 143 (Brinell hardness; suitable for machining and drilling)
- Density: 7.85 g/cm³ (standard for carbon steel calculations)
3. Manufacturing Process Overview
- Steelmaking: Basic oxygen furnace (BOF) or electric arc furnace (EAF) with continuous casting to produce slabs/billets, ensuring consistent chemical composition and low impurity levels.
- Hot Rolling: Heated to 1200-1250°C and rolled into plates, sections, or bars; controlled cooling to prevent internal stresses and ensure uniform grain structure.
- Normalizing (Optional): Heated to 890-950°C and air-cooled to refine grain structure, improving mechanical properties for critical applications.
- Cold Forming: For thin sheets or profiles, cold rolling or bending to achieve precise dimensions and surface finish (e.g., for automotive panels or appliances).
- Surface Treatment: Shot blasting, pickling, or coating (e.g., galvanizing) to enhance corrosion resistance and prepare for painting or further processing.
4. Key Application Fields
Construction & Infrastructure
Structural beams, columns, and reinforcement bars for buildings, bridges, and highways; compliant with GB 50017 and international structural codes.
Machinery & Equipment
Frames, bases, and components for agricultural machinery, mining equipment, and industrial conveyors — balances strength and machinability.
Automotive & Transportation
Chassis components, trailer frames, and railway wagon parts; often galvanized for enhanced durability in outdoor environments.
General Fabrication
Metal furniture, storage racks, and custom fabricated parts; easily welded and formed for prototyping or mass production.
5. Comparison with Similar Carbon Steel Grades
| Grade | Standard | Yield Strength (MPa) | Key Advantage | Typical Use |
|---|---|---|---|---|
| Q255 | GB/T 700 | ≥ 255 | Balanced strength and ductility; cost-effective | General structural and fabrication applications |
| Q235 | GB/T 700 | ≥ 235 | Higher ductility; easier to form and weld | Light structural and non-critical components |
| Q275 | GB/T 700 | ≥ 275 | Higher strength; better load-bearing capacity | Heavy-duty structures and machinery parts |
| ASTM A36 | ASTM A36/A36M | ≥ 250 | Similar properties; widely recognized internationally | Global construction and industrial projects |
6. Selection and Usage Guidelines
- Welding Considerations: Suitable for all standard welding methods (SMAW, GMAW, FCAW); preheating not required for thicknesses ≤ 20mm. Use E43xx or E50xx electrodes for optimal results.
- Corrosion Protection: Apply protective coatings (painting, galvanizing) for outdoor or humid environments; untreated Q255 will rust without protection.
- Machining Tips: Use high-speed steel (HSS) or carbide tools; cutting speeds of 20-30 m/min recommended for turning operations.
- Heat Treatment: Normalizing at 890-950°C can restore properties after cold working; avoid quenching to prevent brittleness.
- Quality Verification: Request mill test certificates (MTC) to confirm compliance with GB/T 700 or equivalent standards (e.g., EN 10025-2 S275JR).
7. Request a Carbon Steel Quote
For customized Q255 carbon steel products — including plates, coils, bars, or profiles — contact our team with your specifications. We provide competitive pricing, technical support, and global shipping for bulk orders.