SM400B: Welded Structural Steel with Improved Low-Temperature Toughness
SM400B, designated under the Japanese Industrial Standards (JIS G3106), is a low-alloy structural steel grade optimized for welded constructions requiring enhanced toughness at low temperatures. With a controlled carbon equivalent and alloying elements like manganese and silicon, it offers superior weldability, moderate strength, and resistance to brittle fracture in cold environments. This makes it ideal for bridges, buildings, ships, and heavy machinery. 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 strength and hardenability; ensures good weldability without preheating |
| Manganese (Mn) | 0.60 – 1.50% | Improves deoxidation and toughness; enhances low-temperature impact resistance |
| Silicon (Si) | ≤ 0.55% | Aids in deoxidation during smelting; contributes to strength and oxidation resistance at elevated temperatures |
| Phosphorus (P) | ≤ 0.035% | Controlled impurity; minimizes brittleness and supports ductility |
| Sulfur (S) | ≤ 0.035% | Restricted to prevent hot shortness and ensure sound weld joints |
| Others (e.g., Cu, Ni) | Cu ≤ 0.40%; Ni ≤ 0.30% (optional) | Alloying additions for corrosion resistance and toughness in specific environments |
2. Key Mechanical Properties (Room Temperature)
- Tensile Strength (σb): 400 – 510 MPa (JIS G3106 standard; suitable for moderate load-bearing structures)
- Yield Strength (σ0.2): ≥ 235 MPa (for plates ≤ 16mm; provides reliable structural integrity)
- Elongation (δ): ≥ 21% (high ductility for forming and welding applications)
- Hardness (HB): ≤ 170 (Brinell; machinable with standard tools)
- Charpy V-Notch Impact (CVN): ≥ 27 J at -20°C (improved low-temperature toughness compared to SM400A)
3. Manufacturing Process Characteristics
- Smelting: Basic oxygen furnace (BOF) or electric arc furnace (EAF) followed by ladle refining to control inclusions and achieve low carbon equivalent (CE ≤ 0.40) for optimal weldability.
- Hot Rolling: Controlled rolling at 850-950°C with finishing temperatures above Ar3 line to refine grain structure and enhance toughness; normalized or air-cooled.
- Heat Treatment: Normalizing at 880-950°C for stress relief and uniform microstructure; optional quenching and tempering for higher strength variants.
- Cold Working: Limited to thin sections; cold bending with radii ≥ 2t and annealing if required to restore ductility.
- Surface Treatment: Shot blasting and priming for corrosion protection; ultrasonic testing for internal defects in thick plates.
4. Typical Application Fields
Bridge and Infrastructure Construction
Bridge girders, highway barriers, and structural beams — excels in welded assemblies exposed to dynamic loads and varying temperatures.
Shipbuilding and Offshore Structures
Hull plates, decks, and marine platforms — provides toughness in cold sea environments without excessive brittleness.
Heavy Machinery and Equipment
Crane booms, excavator frames, and pressure vessels — balances strength, weldability, and fatigue resistance.
Building and General Fabrication
Steel frames for warehouses, workshops, and multi-story buildings — cost-effective for non-corrosive, moderate-stress applications.
5. Difference from Similar Grades (SM400B vs SM400A vs SM490B)
| Grade | Yield Strength (MPa) | Key Advantage | Suitable Scenario |
|---|---|---|---|
| SM400B | ≥ 235 | Enhanced low-temp toughness; good weldability | Cold climates, welded structures like bridges and ships |
| SM400A | ≥ 235 | Standard toughness; lower cost | Mild environments, general fabrication without extreme cold |
| SM490B | ≥ 325 | Higher strength; similar weldability | High-load applications like heavy cranes and pressure equipment |
6. Selection & Usage Precautions
- Weldability Considerations: CE ≤ 0.40 allows welding without preheating up to 20mm thickness; use low-hydrogen electrodes (e.g., E5015) to prevent cracking.
- Low-Temperature Use: Certified for -20°C service; for colder applications (-40°C), opt for higher-toughness grades like SM490C or consult impact testing data.
- Corrosion Protection: Not inherently corrosion-resistant; apply galvanizing or coatings for outdoor exposure to moisture and salts.
- Quality Assurance: Verify compliance with JIS G3106 via mill certificates, including ultrasonic testing (UT) and Charpy impact reports for critical sections.
7. Get a Steel Quote
If you need customized SM400B 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.