Q345q is a low-alloy high-strength steel plate used for welded bridge structures, offering excellent mechanical properties, weldability, and low-temperature toughness. It is widely applied in the main girders and support structures of railway and highway bridges, complying with the Chinese national standard GB/T 714-2015, Structural Steel for Bridge.
Grade Designation
Q345: Indicates the specified minimum yield strength is not less than 345 MPa at room temperature; "Q" stands for "Qu" (yield) in Chinese pinyin.
q: Stands for "Qiao" (bridge) in Chinese pinyin, indicating its use in bridge construction.
C: Denotes quality grade C, requiring an average Charpy V-notch impact energy of no less than 27 J at 0°C, ensuring adequate toughness in cold environments.
This grade is defined in GB/T 714-2015, Structural Steel for Bridge.
Physical Properties
Density: ~7.85 g/cm³
Elastic Modulus: ~210 GPa (at ambient temperature)
Poisson’s Ratio: ~0.3
Coefficient of Thermal Expansion: ~12×10⁻⁶/°C (20–300°C range)
Thermal Conductivity: ~45–50 W/(m·K)
Service Temperature Range: Generally suitable for -20°C to +400°C
Melting Point: ~1500°C (estimated, varies slightly with composition)
Chemical Composition (Typical Values, per GB/T 714-2015)
| Element | Mass Fraction (%) | Notes |
|---|---|---|
| C | ≤0.20 | Controlled to ensure good weldability |
| Si | ≤0.50 | Deoxidizer, improves strength |
| Mn | 1.00–1.60 | Enhances strength and hardenability |
| P | ≤0.025 | Harmful element, strictly limited |
| S | ≤0.015 | Causes hot shortness, tightly controlled |
| Alt (Total Aluminum) | ≥0.020 | Grain refinement, improves toughness |
| Nb, V, Ti (Microalloying Elements) | Total ≤0.11 | Added for grain refinement and precipitation strengthening |
Note: This is a low-alloy high-strength steel, typically aluminum-killed calm steel, produced via controlled rolling (TMCP) or normalization to achieve fine grain and high toughness.
Applications
Main girders, trusses, and towers of railway and highway bridges
Critical load-bearing components in long-span bridges, cable-stayed bridges, and suspension bridges
Bridge structures in cold or high-altitude regions (due to C-grade low-temperature toughness)
Nodes and connection parts in heavy-load transportation bridges
Thanks to its high strength, excellent weldability, and fatigue resistance, Q345qC is widely used in modern large-scale welded bridge engineering projects.
Testing and Manufacturing Methods
Manufacturing Process:
Steel is produced by basic oxygen furnace or electric arc furnace, followed by ladle refining (e.g., LF, RH) to ensure high purity.
Aluminum-killed calm steel, often microalloyed with Nb, V, or Ti for enhanced performance.
Delivered in hot-rolled, TMCP (Thermo-Mechanically Controlled Process), or normalized condition.
Ultrasonic testing (UT) is required for thick plates per GB/T 2970 or JB/T 4730.3 to detect internal defects.
Testing Procedures and Standards:
Tensile Test: GB/T 228.1 – measures yield strength, tensile strength, and elongation.
Impact Test: GB/T 229 – Charpy V-notch test at 0°C; average of three specimens ≥27 J.
Bending Test: GB/T 232 – evaluates ductility and weld joint integrity.
Chemical Analysis: GB/T 20123 or GB/T 223 series standards.
Ultrasonic Testing: GB/T 2970 or JB/T 4730.3, typically for plates ≥6 mm thick.
Equivalent or Similar Grades in Other Standards
| Standard System | Similar Grade | Notes |
|---|---|---|
| ASTM (USA) | ASTM A709 Grade 50 or A572 Grade 50 | Both have a minimum yield strength of 345 MPa; widely used in U.S. bridge construction |
| JIS (Japan) | SM490B or SMA570W | SM490B for general structures; SMA570W is weathering steel for bridges, strength level similar |
| EN (Europe) | S355J2+N (EN 10025-3) | European structural steel with 355 MPa yield strength and good weldability; commonly used in bridge applications |
Note: While mechanical properties are comparable, direct substitution requires engineering evaluation, welding procedure qualification, and compliance with project specifications.
The grade Q345q was first introduced in GB/T 714-2008, Structural Steel for Bridge.
Published on September 11, 2015, and implemented on June 1, 2016, this standard replaced the previous GB/T 714-2000, Weathering Steel for Welded Structures. The new version expanded the scope from weathering steels to general structural steels for bridge applications and introduced a series of "q" (for "Qiao" – bridge) grades.
Q345q has since become one of the most widely used bridge steels in China, marking a significant advancement in the standardization and systematization of bridge construction materials in the country.
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.
