1. Core Processing Principles & Microstructural Effects
Hot-rolled SPA-H coil: Processed above the steel's recrystallization temperature (typically 900–1200°C). The high temperature allows the steel grains to recrystallize and grow uniformly, resulting in a coarse, equiaxed ferrite-pearlite microstructure with low internal stress.
Cold-rolled SPA-H coil: Processed at room temperature, below the recrystallization temperature. The plastic deformation during rolling causes grain elongation, dislocation accumulation, and work hardening of the steel, which significantly increases its strength but reduces ductility.
2. Tensile Strength Comparison (Compliant with JIS G 3115)
| Coil Type | Tensile Strength Range | Yield Strength Reference | Key Notes |
|---|---|---|---|
| Hot-rolled SPA-H coil | ≥ 480 MPa | ≥ 345 MPa | No work hardening; uniform microstructure ensures balanced strength and ductility (elongation ≥ 20%), suitable for structural components (e.g., building frames, containers). |
| Cold-rolled SPA-H coil | 550–650 MPa | ≥ 450 MPa | Work hardening effect boosts tensile strength by 15–35% compared to hot-rolled coils; elongation decreases (typically 10–15%), suitable for precision parts requiring high strength (e.g., decorative panels, small structural fittings). |
3. Key Factors Affecting the Strength Gap
Work hardening: The primary driver of higher strength in cold-rolled coils-plastic deformation at room temperature introduces internal stress and refines the grain structure, which resists further deformation under tensile loads.
Post-processing adjustment: Cold-rolled SPA-H coils can be annealed to reduce strength (restoring ductility) if needed; annealed cold-rolled coils have tensile strength close to hot-rolled levels (≈ 490–520 MPa).
