Q550q is a high-strength low-alloy steel plate specified in GB/T 714 for bridge structures, offering excellent strength, toughness, weldability, and atmospheric corrosion resistance, suitable for heavy-load and large-span bridges.
Grade Designation Explanation
Q: Stands for "Yield Strength" (Qu Fu) in Chinese pinyin;
550: Indicates the minimum yield strength of 550 MPa;
q: Represents "Bridge" (Qiao) in Chinese pinyin, signifying that this grade is specifically intended for bridge construction.
Physical Properties
Density: ~7.85 g/cm³
Elastic Modulus: ~206 GPa
Thermal Expansion Coefficient: ~12×10⁻⁶/°C (20–300°C)
Thermal Conductivity: ~40–50 W/(m·K)
Yield Strength (ReL): ≥550 MPa
Tensile Strength (Rm): 670–830 MPa
Elongation (A₅₀mm): ≥16%
Impact Toughness: KV₂ ≥47 J at –40°C (typical requirement for Q550q in cold regions)
Chemical Composition (Typical, wt%)
C: ≤0.18
Si: 0.20–0.60
Mn: 1.20–1.70
P: ≤0.025
S: ≤0.015
Nb: 0.01–0.05
V: 0.02–0.10
Ti: 0.01–0.05
Alt: ≥0.020
(Note: Microalloyed with Nb, V, Ti for grain refinement and precipitation strengthening.)
Application Areas
Used in the fabrication of critical components in modern steel bridges, especially under heavy loads, low temperatures, or corrosive environments:
Main girders and trusses of long-span bridges;
Cable-stayed bridge towers and anchorages;
Orthotropic steel decks;
Seismic-resistant bridge structures in cold regions.
Testing and Manufacturing Methods
Manufacturing Process: Produced via oxygen converter or electric furnace melting, followed by ladle refining (LF), degassing (VD/RH), continuous casting, and controlled rolling (TMCP) or quenching and tempering (QT).
Heat Treatment: Typically TMCP or QT to achieve high strength and good toughness.
Testing: Tensile, bend, impact (–40°C), Z-direction tensile, ultrasonic testing (UT), weldability assessment, and fatigue testing.
Standard: GB/T 714-2015 "Steel Plates for Bridge Applications".
Equivalent or Similar Grades in ASTM, JIS, and EN
China (GB): Q550q
USA (ASTM): ASTM A709 Grade 100
Japan (JIS) : JIS G3116: SMHA80
Europe (EN): EN 10025-6: S550Q/S550QL1
Note: Direct equivalence may vary; engineering evaluation is required for cross-standard substitution.
First Appearance in Standard and Version
The Q550q grade was first introduced in GB/T 714-2015 "Steel Plates for Bridge Applications", which replaced GB/T 714-2000. This edition significantly upgraded the strength levels and performance requirements for bridge steels in China, introducing high-strength grades like Q420q, Q460q, Q500q, and Q550q to support the construction of large-scale, high-performance bridges.
Ultrasonic Testing (UT)
A key non-destructive testing technique that uses high-frequency sound waves to detect internal flaws in steel plates. The probe emits sound waves, which reflect when encountering defects such as cracks or inclusions. The receiver captures the echoes, enabling precise determination of defect location and size. With high sensitivity, strong penetration, and fast inspection speed, UT effectively ensures internal quality, widely used in the production of heavy plates, pressure vessel plates, and other high-end products to guarantee safety and reliability.
Magnetic Particle Testing (MT)
A common surface inspection method that magnetizes the workpiece, causing leakage magnetic fields at surface or near-surface defects like cracks or inclusions, which attract magnetic particles to form visible indications. Simple to operate and highly sensitive, MT is suitable for rapid inspection of surface and near-surface flaws in ferromagnetic materials, widely used for online or offline inspection of plate edges, ends, and welds, ensuring product quality and safety.
Penetrant Testing (PT)
A non-destructive method for detecting surface-breaking flaws. A penetrant liquid is applied to the cleaned steel surface, allowing it to seep into defects such as cracks or pores. After removing excess penetrant, a developer is applied, causing the trapped penetrant to bleed out and form visible indications. Simple and cost-effective, PT is suitable for inspecting surface defects in various non-porous materials, commonly used for welds, castings, and complex components, effectively ensuring surface quality of steel plates.
