Jis G3125 Spa H
JIS G3125 SPA-H steel is a high-strength, low-alloy structural steel with:
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Balanced chemical composition for toughness and weldability
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Mechanical properties suitable for bridges, ships, and industrial frames
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Common applications in construction, shipbuilding, and heavy machinery
- Description
JIS G3125 SPA-H is a high-strength, low-alloy structural steel standard widely used in construction, shipbuilding, and industrial engineering. It is the Japanese Industrial Standard (JIS) equivalent of high-strength structural plates, designed for high toughness, weldability, and structural reliability.
1. Standard Overview
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Standard: JIS G3125
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Grade: SPA-H (Structural Plate – High strength)
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Type: Low-alloy, high-strength steel plate suitable for welded structures
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Focus: Combines high yield strength with good toughness at ambient temperatures
2. Chemical Composition
| Element | Content (%) | Function |
|---|---|---|
| Carbon (C) | 0.12 – 0.18 | Provides strength |
| Manganese (Mn) | 1.0 – 1.5 | Improves toughness and hardenability |
| Phosphorus (P) | ≤ 0.035 | Impurity control |
| Sulfur (S) | ≤ 0.035 | Impurity control |
| Silicon (Si) | ≤ 0.50 | Enhances strength and oxidation resistance |
| Copper (Cu) | ≤ 0.25 | Slightly improves corrosion resistance |
| Nickel (Ni) | Trace | Improves toughness in some cases |
| Chromium (Cr) | Trace | Contributes to strength and surface quality |
Key Feature: Low-alloy composition ensures high strength, weldability, and good cold formability, while maintaining toughness.
3. Mechanical Properties
| Property | Typical Value |
|---|---|
| Yield Strength (MPa) | ≥ 355 |
| Tensile Strength (MPa) | 470 – 630 |
| Elongation (%) | ≥ 20 |
| Impact Toughness | Good at ambient temperature |
| Hardness (HB) | 140 – 190 |
Highlights:
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High load-bearing capacity for structural use
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Good ductility for forming and bending
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Weldable without preheating in most applications
4. Key Applications
| Industry | Typical Use Cases |
|---|---|
| Construction | Bridges, structural beams, trusses, building frameworks |
| Shipbuilding | Hull plates, decks, offshore platform structures |
| Industrial & Engineering | Machinery frames, storage tanks, pipelines |
| Heavy Equipment | Cranes, load-bearing frames, structural supports |
Advantages in Use:
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High strength allows thinner sections for same load
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Toughness ensures safety under dynamic loads
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Weldable for complex structural fabrication
5. Comparison with Other Structural Steels
| Grade | Yield Strength (MPa) | Primary Use |
|---|---|---|
| SPA-H (JIS G3125) | ≥355 | High-strength welded structural plates |
| SPA-C | ≥315 | Medium-strength structural plates |
| S355 (EN) | 355 | European structural equivalent |
| ASTM A572 | 345–450 | High-strength structural steel |
Observation: SPA-H is widely used in Japan and Asia for structural and shipbuilding plates where strength, weldability, and reliability are critical.
