SM400A: Low-Carbon Structural Steel with Excellent Weldability
SM400A, specified under the Japanese Industrial Standard (JIS G3106), is a low-carbon structural steel grade renowned for its superior weldability, moderate strength, and cost-effectiveness. With a carbon content limited to 0.25%, it offers good formability and toughness, making it ideal for welded structures in construction, machinery, and transportation sectors. This article explores its chemical composition, mechanical properties, manufacturing processes, applications, and comparisons with related grades.
1. Core Chemical Composition (JIS G3106 Standard)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | ≤ 0.25% | Limits strength while enhancing weldability and ductility |
| Manganese (Mn) | 0.60 – 1.50% | Improves hardenability and toughness; deoxidizer in steelmaking |
| Phosphorus (P) | ≤ 0.035% | Controlled impurity; minimizes brittleness and cold shortness |
| Sulfur (S) | ≤ 0.035% | Restricted to prevent hot shortness and improve machinability |
| Silicon (Si) | ≤ 0.55% | Acts as a deoxidizer; enhances strength and oxidation resistance |
2. Key Mechanical Properties (Room Temperature)
- Tensile Strength (σb): 400 – 510 MPa (JIS G3106 standard for plates ≥16mm thickness)
- Yield Strength (σ0.2): ≥ 235 MPa (adequate for structural loading in mild conditions)
- Elongation (δ): ≥ 21% (supports forming and bending operations)
- Hardness (HB): ≤ 170 (Brinell; facilitates machining and welding)
- Impact Toughness (Charpy V-notch): ≥ 27 J at 0°C (reliable performance in moderate-temperature environments)
3. Manufacturing Process Characteristics
- Smelting: Basic oxygen furnace (BOF) or electric arc furnace (EAF) with ladle refining (LF) to achieve low impurity levels and uniform composition.
- Hot Rolling: Performed at 850-1100°C, followed by controlled cooling to normalize microstructure and ensure fine grain size.
- Heat Treatment: Normalizing at 880-950°C optional for enhanced toughness; air cooling to avoid excessive hardness.
- Cold Forming: Suitable for plates up to 50mm; preheating not required for welding due to low carbon equivalent (CE ≤ 0.40).
- Surface Treatment: Shot blasting to remove scale; optional primer coating for corrosion protection in outdoor applications.
4. Typical Application Fields
Construction & Infrastructure
Building frameworks, bridges, steel towers, and platforms — provides reliable structural integrity with easy on-site welding.
Machinery & Equipment
Heavy machinery bases, conveyor frames, agricultural implements, and pressure vessels — balances strength with fabricability.
Transportation
Railway components, shipbuilding hull sections, and truck chassis — excels in weld-intensive assemblies.
General Fabrication
Piping supports, storage silos, and industrial racks — cost-effective for non-critical, welded structures.
5. Difference from Similar Grades (SM400A vs SM400B vs SM490A)
| Grade | Yield Strength | Key Advantage | Suitable Scenario |
|---|---|---|---|
| SM400A | ≥ 235 MPa | Superior weldability; low cost | General welded structures in mild environments |
| SM400B | ≥ 235 MPa | Guaranteed low-temperature toughness (-20°C impact) | Cold-climate applications like northern infrastructure |
| SM490A | ≥ 265 MPa | Higher strength for demanding loads | Heavy-duty beams, bridges, and high-stress frames |
6. Selection & Usage Precautions
- Environmental Limitations: Suitable for atmospheric exposure; use weathering steels (e.g., SMA400) for severe corrosion or marine settings.
- Welding Guidelines: Preheat unnecessary for thicknesses <20mm; employ low-hydrogen electrodes (e.g., E7018) to prevent cracking.
- Thickness Considerations: Properties vary by plate thickness; verify for >50mm to ensure consistent yield strength.
- Quality Assurance: Obtain material certificates per JIS G3106; perform ultrasonic testing for internal defects in critical applications.
7. Get a Structural Steel Quote
If you need customized SM400A steel products (plates, sections, bars) or want to get a detailed price quote, please contact our team directly. Our specialists will provide professional technical support and competitive pricing based on your requirements.