Q235 Carbon Steel: Mild Steel Grade for Structural and General Engineering Applications
Q235 carbon steel, designated under the Chinese GB/T 700 standard (equivalent to ASTM A36, EN S235JR, and JIS SS400), is the most widely used mild steel grade in structural engineering, manufacturing, and construction. Characterized by its balanced mechanical properties, excellent weldability, and cost-effectiveness, Q235 serves as a foundational material for bridges, buildings, machinery, and automotive components. This article explores its chemical composition, mechanical properties, processing methods, application scenarios, and comparative advantages over similar grades.
1. Chemical Composition (GB/T 700 Standard)
| Element | Content Range (wt%) | Function |
|---|---|---|
| Carbon (C) | ≤ 0.22% | Primary strengthening element; low content ensures ductility and weldability |
| Manganese (Mn) | 0.30 – 0.65% | Enhances strength and deoxidation; improves hardenability |
| Silicon (Si) | ≤ 0.30% | Deoxidizer; improves strength without sacrificing ductility |
| Sulfur (S) | ≤ 0.050% | Impurity; controlled to prevent hot shortness during rolling |
| Phosphorus (P) | ≤ 0.045% | Impurity; limited to maintain toughness and cold-forming properties |
| Chromium (Cr) | ≤ 0.30% | Residual element; minimal impact on properties at this level |
| Nickel (Ni) | ≤ 0.30% | Residual element; negligible effect on mechanical performance |
| Copper (Cu) | ≤ 0.30% | Residual; may improve atmospheric corrosion resistance slightly |
2. Mechanical Properties (Room Temperature)
- Tensile Strength (σb): 370 – 500 MPa (varies by thickness; higher for thinner sections)
- Yield Strength (σs): ≥ 235 MPa (minimum guaranteed value; defines structural load capacity)
- Elongation (δ₅): ≥ 26% (for thickness ≤ 16mm; ensures formability for bending/rolling)
- Impact Energy (AKV): ≥ 27 J at 20°C (tested via Charpy V-notch; indicates toughness)
- Hardness (HB): ≤ 137 (Brinell; soft enough for machining and cold working)
- Density: 7.85 g/cm³ (standard for carbon steel; critical for weight calculations)
3. Manufacturing and Processing Methods
- Steelmaking: Basic oxygen furnace (BOF) or electric arc furnace (EAF) with continuous casting; aluminum or silicon killed for fine grain structure.
- Hot Rolling: Slabs reheated to 1100-1250°C and rolled into plates/coils; controlled cooling to prevent internal stresses.
- Cold Rolling: For thin sheets (≤ 3mm); reduces thickness while improving surface finish (e.g., SPCC equivalent).
- Heat Treatment:
- Normalizing: 880-940°C + air cooling to refine grain size and improve machinability.
- Annealing: 600-700°C for stress relief after cold working.
- Surface Treatment:
- Pickling (HCl/H₂SO₄) to remove oxide scale.
- Oiling or phosphating for temporary corrosion protection.
- Hot-dip galvanizing (Z120-Z275) for outdoor applications.
4. Key Application Fields
Construction & Infrastructure
Structural beams (I-beams, H-beams), reinforcement bars, bridge components, and prefabricated buildings. Compliant with GB 50017-2017 for steel structures.
Machinery & Equipment
Gears, shafts, and frames for agricultural/industrial machinery; base plates for CNC machines. Often paired with Q345 for welded assemblies.
Automotive & Transportation
Chassis components, truck beds, and railway wagon frames. Used in non-critical parts where high strength is unnecessary.
Pipe & Tube Manufacturing
Water/gas pipelines (GB/T 3091), structural tubing, and scaffolding. Often galvanized (e.g., DN15-DN150 pipes).
Shipbuilding & Offshore
Hull structures for inland vessels, barges, and platform decks (CCS/ABS Grade A equivalent).
General Fabrication
Metal furniture, storage racks, and enclosures. Easily cut, drilled, and welded for custom fabrication.
5. Comparison with Similar Carbon Steel Grades
| Grade | Standard | Yield Strength (MPa) | Key Features | Typical Use |
|---|---|---|---|---|
| Q235 | GB/T 700 | ≥ 235 | Balanced properties; cost-effective; excellent weldability | General structural and non-critical components |
| ASTM A36 | ASTM A36/A36M | 250 (min) | Similar to Q235 but with stricter Mn content (0.8-1.2%) | US/export markets; bridges and buildings |
| S235JR | EN 10025-2 | 235 (min) | European equivalent; guaranteed impact at 20°C (JR) | EU structural projects; CE-marked products |
| SS400 | JIS G3101 | 245 (min) | Japanese standard; no specified chemical composition | Asian markets; shipbuilding and construction |
| Q235B | GB/T 700 | ≥ 235 | Stricter S/P control (≤ 0.045/0.045%); better toughness | Welded structures; cold-forming applications |
6. Selection Guidelines and Precautions
- Thickness Considerations: Yield strength drops for thicker plates (>16mm); verify mechanical properties via mill test certificates (MTC).
- Welding: Use E43xx (AWS A5.1) electrodes; preheat not required for thickness ≤ 20mm. Post-weld stress relief recommended for constrained joints.
- Corrosion Protection:
- Indoor: Oiled or painted surfaces suffice.
- Outdoor: Hot-dip galvanizing (≥ 80µm coating) or zinc-rich primers.
- Marine: Avoid prolonged exposure; upgrade to Q345 or cortical steel.
- Machining: Low carbon content enables high-speed machining; use carbide tools for prolonged tool life. Surface hardness ≤ 160 HB.
- Quality Verification: Request:
- Chemical analysis (spectrometer test report).
- Tensile/test certificates (EN 10204 3.1/3.2).
- Ultrasonic testing (UT) for critical applications.
- Substitutes: For higher strength, consider Q345 (≈ A572 Gr.50); for better formability, use DC01 (cold-rolled steel).
7. Request a Carbon Steel Quote
Need Q235 carbon steel in custom sizes (plates, coils, bars, or profiles)? Contact Baoli Iron & Steel for competitive pricing, technical specifications, and lead times. Our team provides:
- Mill-direct sourcing with GB/T 700 certification.
- Cut-to-length, slitting, and surface treatment services.
- Export packaging (bundled, wooden crates, or palletized).