SM490B: Welded Structural Steel with Optimized Low-Temperature Impact Toughness
SM490B, a high-strength low-alloy (HSLA) structural steel grade under the Japanese JIS G3106 standard, is engineered for welded constructions requiring superior toughness at low temperatures. With a minimum yield strength of 325 MPa and enhanced impact properties down to -20°C, it offers excellent weldability, formability, and resistance to brittle fracture, making it ideal for heavy machinery, bridges, and offshore structures. This article explores its chemical composition, mechanical properties, manufacturing processes, applications, and comparative advantages.
1. Core Chemical Composition (JIS G3106 Standard)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | ≤ 0.20% | Limits hardenability; ensures good weldability without preheating |
| Silicon (Si) | ≤ 0.55% | Improves strength and oxidation resistance; aids in deoxidation |
| Manganese (Mn) | 0.60 – 1.50% | Enhances toughness and strength; stabilizes microstructure |
| Phosphorus (P) | ≤ 0.035% | Controlled to minimize brittleness and improve ductility |
| Sulfur (S) | ≤ 0.035% | Restricted to prevent hot shortness and ensure weld integrity |
| Chromium (Cr) | Optional ≤ 0.45% | Boosts hardenability and corrosion resistance in alloyed variants |
| Niobium (Nb) + Vanadium (V) + Titanium (Ti) | ≤ 0.30% | Microalloying elements for grain refinement and precipitation strengthening |
2. Key Mechanical Properties (Room Temperature)
- Yield Strength (ReH): ≥ 325 MPa for t ≤ 16 mm (high load-bearing capacity for structural elements)
- Tensile Strength (Rm): 490 – 610 MPa (balanced strength without excessive brittleness)
- Elongation (A): ≥ 17% (adequate ductility for fabrication and forming)
- Charpy V-Notch Impact Toughness: ≥ 47 J at -20°C (optimized for cold climates and dynamic loading)
- Hardness (HB): ≤ 170 (machinable and suitable for welding without cracking)
3. Manufacturing Process Characteristics
- Steelmaking: Basic oxygen furnace (BOF) or electric arc furnace (EAF) followed by ladle refining and vacuum degassing to achieve low impurity levels and fine grain structure.
- Hot Rolling: Controlled rolling at 850-950°C with finish rolling above 800°C, followed by air cooling or normalized to refine ferrite-pearlite microstructure.
- Heat Treatment: Normalizing at 880-950°C for 1 hour per 25 mm thickness, air cooling to enhance toughness; optional TMCP (thermomechanical controlled processing) for superior properties.
- Cold Forming: Limited to thicknesses up to 20 mm; requires low strain rates to avoid strain aging and maintain impact toughness.
- Testing & Finishing: Ultrasonic testing for internal defects; surface inspection per JIS G3193; pickling or shot blasting for scale removal.
4. Typical Application Fields
Bridge & Civil Engineering
Girders, trusses, and crane runway beams — withstands seismic loads and low-temperature exposure in harsh environments.
Heavy Machinery
Construction equipment frames (excavators, bulldozers), mining machinery chassis, and pressure vessels — reliable under cyclic and impact stresses.
Offshore & Marine Structures
Platform legs, shipbuilding hull sections, and coastal defenses — resists low-temperature brittleness in sub-zero conditions.
Industrial Fabrication
Welded storage tanks, conveyor frames, and agricultural machinery — cost-effective alternative to higher-grade steels in moderate-duty applications.
5. Difference from Similar Grades (SM490B vs SM490A vs SM490C)
| Grade | Impact Test Temperature | Key Advantage | Suitable Scenario |
|---|---|---|---|
| SM490A | Room temperature (≥ 27 J) | General-purpose; lower cost | Mild climates; non-critical structures |
| SM490B | -20°C (≥ 47 J) | Enhanced low-temperature toughness | Cold regions; welded dynamic-load applications |
| SM490C | -0°C (≥ 47 J) | Balanced toughness for transitional climates | Temperate zones; moderate impact requirements |
6. Selection & Usage Precautions
- Welding Guidelines: Suitable for all-position welding with E7018 electrodes; preheating unnecessary up to 20 mm thickness, but post-weld heat treatment recommended for thicknesses >40 mm to relieve stresses.
- Corrosion Considerations: Provides basic atmospheric resistance; for marine or acidic environments, apply protective coatings or consider weathering steels like SM570.
- Design Limits: Minimum service temperature -20°C; avoid use below -40°C without additional toughness verification via Charpy testing.
- Quality Assurance: Specify JIS G3106 certification and request material certificates (e.g., EN 10204 3.1) to verify composition and properties.
7. Get a Structural Steel Quote
If you need customized SM490B steel products (plates, sections, beams, or coils) 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.