1.What are the differences between the two in terms of brand meaning and core strength?
Both grades are designated as "Q + minimum yield strength + quality grade" (some grades without a suffix assume the base grade). The core difference lies primarily in yield strength:
Q345B: "Q345" indicates a yield strength of at least 345 MPa; "B" represents the quality grade (corresponding to the -20°C impact toughness requirement).
Q390: "Q390" indicates a yield strength of at least 390 MPa (approximately 13% higher than Q345B); quality grades also include A, B, and C (e.g., Q390B corresponds to the -20°C impact toughness requirement).
2.What are the differences in mechanical properties between the two?
Yield strength: Q390 is approximately 13% higher, directly determining its upper load-bearing capacity.
Tensile strength: Q390 has a higher tensile strength range (approximately 21%-14% higher than Q345B), consistent with the increasing yield strength trend.
Elongation: Q390 has slightly lower elongation (increased strength is typically accompanied by a slight decrease in plasticity), but still meets the structural "plastic deformation requirements."
Impact toughness: Within the same quality grade (e.g., Grade B), impact toughness requirements are consistent (the standard mandates low-temperature toughness to avoid brittle failure).
3.What is the difference in weldability between the two?
The carbon equivalent of Q345B is typically ≤0.45%, offering excellent weldability. Conventional welding (such as arc welding and submerged arc welding) does not require complex preheating (generally unnecessary for ambient temperatures ≥0°C), making it suitable for on-site batch welding.
Q390's carbon equivalent is slightly higher than Q345B (typically ≤0.48%) due to the addition of alloying elements. It also exhibits a slightly greater tendency to harden, requiring stricter process control during welding. For example, preheating to 50-100°C is required for low-temperature environments (≤0°C). Welding thick plates (≥25mm) may require higher preheat temperatures, and controlled weld energy input is essential to avoid hot and cold cracking.
4.What is the difference in cold working/hot working properties between the two?
Cold Working (e.g., curling, bending): Both Q390 and Q390 offer good cold workability, but due to their higher strength and slightly lower ductility, the bend radius must be controlled during cold bending (to avoid cracking), and preheating may be necessary for larger bend angles.
Hot Working (e.g., rolling, forging): The process differs little, and both are suitable for conventional hot working processes.
5.What are the differences in their application scenarios?
Q345B: The "best choice for cost-effectiveness," suitable for most common applications.
Due to its moderate strength, excellent weldability, and relatively low price (5%-10% lower than Q390), it is currently the most widely used low-alloy steel in construction and engineering. Its primary applications include:
Frame beams/columns for high-rise buildings, long-span roof trusses, and crane beams for industrial plants (for medium-load applications);
Small and medium-sized bridges (spans ≤ 30m), and supports for municipal engineering projects (such as streetlight poles and signal towers);
Steel-concrete composite structures (such as composite beams and concrete-filled steel tubular columns), and other cost-sensitive, high-volume construction applications.
Q390: "High-Strength Demand Scenarios," focusing on "weight reduction and efficiency improvement."
Due to its higher strength, it is suitable for scenarios with high loads and the need for controlled component cross-sections. It can achieve weight reduction or space savings by reducing steel usage or reducing cross-sections. Its primary applications include:
Core tubes of super-high-rise buildings (e.g., steel columns in buildings over 150m tall), main beams of long-span bridges (spans of 30-50m);
Heavy equipment supports (e.g., track beams for cranes over 100t); and specialized structures (e.g., steel hulls of large storage tanks and auxiliary structures for offshore platforms);
Weight-sensitive structures (e.g., steel box girders for bridges and load-bearing components of prefabricated buildings), which can reduce component deadweight by 10%-15%.