Q355B Carbon Steel
Q355B is a widely used low-alloy high-strength structural steel known for its excellent mechanical properties, weldability, and versatility in engineering applications. It belongs to China’s GB/T standard series and is designed for structural components requiring higher strength than ordinary carbon steels (e.g., Q235), making it a staple in construction, machinery, and automotive industries.
Basic Information
Definition: Q355B is a low-alloy steel (with alloying elements like manganese, silicon, and trace amounts of niobium, vanadium, or titanium) that achieves high strength through solid solution strengthening and grain refinement. The “B” in its designation indicates it meets impact toughness requirements at 20°C, ensuring reliability in moderate-temperature environments.
Corresponding Standards: Conforms to GB/T 1591-2018 (Low-alloy high-strength structural steels). It is equivalent to international grades such as:
European: S355JR (EN 10025-2)
American: ASTM A572 Grade 50
Japanese: JIS SM490B
Key Characteristics: High yield strength, good toughness, excellent weldability, and cost-effectiveness, making it suitable for load-bearing structural components.
Chemical Composition
Q355B’s composition is balanced to optimize strength, toughness, and weldability, with controlled alloying elements:
Element Content Range Role in the Steel
Carbon (C) ≤0.20% Provides strength while keeping weldability intact (low carbon avoids brittle welds).
Manganese (Mn) 1.00–1.60% Primary strengthening element; enhances toughness and hardenability.
Silicon (Si) ≤0.55% Acts as a deoxidizer; improves strength and oxidation resistance.
Phosphorus (P) ≤0.035% Strictly controlled to prevent brittleness, especially in welded joints.
Sulfur (S) ≤0.035% Minimized to avoid hot cracking during welding and reduce toughness loss.
Niobium (Nb) 0.015–0.060% Refines grain structure, improving strength and toughness.
Vanadium (V) 0.02–0.15% Enhances strength through grain refinement and precipitation hardening.
Titanium (Ti) 0.02–0.20% Refines grains and prevents excessive grain growth during heat treatment.
Iron (Fe) Balance Base metal providing structural integrity.
Physical Properties
Density: ~7.85 g/cm³ (consistent with most carbon and low-alloy steels).
Melting Point: ~1430–1450°C.
Thermal Conductivity: ~45 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~12×10⁻⁶/°C (20–100°C), suitable for structural stability in temperature fluctuations.
Elastic Modulus: ~206 GPa, ensuring rigidity in load-bearing applications.
Mechanical Properties
Q355B’s mechanical properties are defined by its high strength and good toughness, critical for structural safety:
Property Minimum Requirement (Hot-Rolled State)
Yield Strength (Rp0.2) ≥355 MPa (hence the designation “Q355”)
Tensile Strength (Rm) 470–630 MPa
Elongation (A) ≥21%
Impact Toughness (Akv) ≥34 J (at 20°C, V-notch test)
Hardness ~140–180 HB
Toughness: The “B” grade ensures it retains sufficient impact resistance at 20°C, making it suitable for outdoor and industrial environments.
Fatigue Resistance: Good for cyclic load applications (e.g., bridges, machinery frames) due to its balanced strength and ductility.
Welded Joint Performance: Welds retain ~80–90% of the base metal’s strength when proper welding procedures are used.
Heat Treatment
Q355B is typically used in the hot-rolled or normalized state and does not require additional heat treatment for most applications:
Normalizing: Optional for large sections to refine grain structure and improve uniformity. Process: Heat to 880–920°C, hold, then air cool.
Welding Heat Treatment: Post-weld annealing may be used for thick sections to reduce residual stresses, but it is not mandatory for most structural uses.
Processing Performance
Weldability: Excellent—compatible with common welding methods (arc welding, gas welding, resistance welding). Low carbon and controlled impurities minimize the risk of cold cracking. Preheating is generally not required for thin sections (<25mm). Formability: Good for cold and hot forming processes, including bending, rolling, and punching. Its ductility allows for complex shapes in structural components (e.g., beams, pipes). Machinability: Suitable for standard machining operations (turning, milling, drilling) using high-speed steel or carbide tools. Its strength requires proper tool selection to avoid excessive wear. Cutting: Easily cut with plasma, laser, or oxy-fuel methods, producing clean edges suitable for structural assembly. Application Fields Q355B is the material of choice for load-bearing structural components across industries: Construction Engineering: Bridges, building frames, and steel structures (columns, beams, trusses). High-rise buildings and industrial plants requiring high strength-to-weight ratios. Mechanical Manufacturing: Machinery frames, equipment bases, and structural parts for heavy machinery. Crane booms, hydraulic supports, and pressure vessel shells (non-critical pressure applications). Automotive & Transportation: Truck frames, trailer chassis, and bus structural components. Railway vehicle underframes and cargo containers. Energy & Infrastructure: Oil and gas pipelines, power plant structures, and wind turbine towers. Transmission towers and communication masts. Advantages and Limitations Advantages High Strength-to-Cost Ratio: Offers 50% higher yield strength than Q235 at a moderate price premium. Versatility: Suitable for diverse applications from construction to machinery. Easy Fabrication: Welds and forms well, reducing manufacturing complexity. Reliability: Consistent performance in ambient and moderate-temperature environments. Limitations Corrosion Resistance: Poor in humid or corrosive environments; requires protective coatings (painting, galvanizing) for outdoor use. High-Temperature Performance: Not suitable for sustained use above 300°C, as strength decreases significantly. Thick Section Limitations: Very thick plates (>100mm) may require controlled rolling or normalizing to ensure uniform properties.
In summary, Q355B is a reliable low-alloy high-strength steel that balances performance, cost, and fabricability, making it indispensable in modern structural engineering and manufacturing. Its widespread adoption underscores its role as a workhorse material for load-bearing applications.