1. Key Parameter Comparison Table

（1）Grade 480 (70)

• Yield Strength (MPa/ksi)：≥480 (70)

• Tensile Strength (MPa)：620-760

• Elongation (%)：≥16

• Impact Toughness (-40°C)：≥27 J (Normalized)

• Density (g/cm³)：7.85

• Typical Standards：ASTM A514, EN 10025-6 S690QL

（2）Grade 550 (80)

• Yield Strength (MPa/ksi)：≥550 (80)

• Tensile Strength (MPa)：620-830

• Elongation (%)：≥16-18

• Impact Toughness (-40°C)：≥27 J (e.g., Q550D)

• Density (g/cm³)：7.85

• Typical Standards：ASTM A572 Gr.80, EN S550QL

2. Key Differences in Depth

（1）Strength vs. Cost Balance

1. Grade 480 (70)

• Advantages: Offers better cost performance in the 480-550 MPa range.

• Disadvantages: Yield strength is about 15% lower than Grade 550.

2. Grade 550 (80)

• Advantages: Can achieve a 10-15% weight reduction (under the same load conditions).

• Disadvantages: The unit price is about 8-12% higher due to microalloying costs.

（2）Metallurgical Process Differences

1. Grade 480 (70)

• Strengthening Mechanism：Traditional Quenching & Tempering (Q&T)

• Grain Size：ASTM 7-8

• Anisotropy：More pronounced (needs attention to rolling direction)

2. Grade 550 (80)

• Strengthening Mechanism：TMCP + Microalloying (Nb/V)

• Grain Size：ASTM 8-9 (finer)

• Anisotropy：Better controlled

（3）Dynamic Performance Comparison

1. Fatigue Limit:

• Grade 480(70) ≈ 280 MPa (10⁷ cycles)

• Grade 550(80) ≈ 320 MPa (10⁷ cycles)

(Data based on R=-1 axial loading)

2. Crack Growth Rate:

• Grade 550 exhibits a 15-20% lower crack growth rate at ΔK = 20 MPa√m.

3. Typical Application Scenarios

（1）Grade 480 (70) Preferred Scenarios:

• Construction Machinery: Excavator swing platform (balancing strength and cost)

• Ship Structures: Deck reinforcement material (equivalent to ISO 630 EH40)

• Traditional Bridges: Non-large span steel box girders (ASTM A709 Gr.70)

（2）Grade 550 (80) Preferred Scenarios:

• New Energy Equipment: Wind turbine tower sections (EN 10025-6 S550QL)

• Special Transport: Semi-trailer beams (key weight reduction parts)

• Modular Buildings: High-rise steel structure joints (reducing steel usage)

4. Processing Performance Comparison

（1）Grade 480 (70)

• Cold Bending Radius：≥3t (t = plate thickness)

• Welding Heat Input：≤25 kJ/cm

• Machinability：Tool life reduced by ~20%

• Surface Treatment：Needs sandblasting Sa2.5 grade

（2）Grade 550 (80)

• Cold Bending Radius：≥2.5t

• Welding Heat Input：≤30 kJ/cm

• Machinability：Tool life reduced by ~15%

• Surface Treatment：Can be directly phosphated

5. Latest Technological Developments

（1）Grade 480 Improvement:

Baosteel BS700MC enhances elongation to 19% via Ti microalloying.

（2）Grade 550 Upgrade:

SSAB Strenx 550+ achieves -60°C impact toughness (released in 2023).

6. Summary and Recommendations

• Conservative Selection: Prefer Grade 480 (70) — Mature process, stable supply chain

• Innovative Projects: Recommend Grade 550 (80) — Superior overall performance, suitable for lightweight design

• Key Tip: The two materials are not directly interchangeable and require re-evaluation through FEA simulation.

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