Q390B is a low-alloy, high-strength structural steel specified in the Chinese national standard GB/T 1591-2018 (Low-alloy high-strength structural steels). It is a grade one level higher in strength than the common Q355B.
Here's a detailed breakdown of its key characteristics:
1. Core Material Identity
Standard: GB/T 1591-2018.
Grade Designation Decoding:
Q: Stands for Yield Strength (屈, Qu).
390: Indicates the minimum yield strength (ReH) in MPa for the thinnest product form (thickness ≤ 16 mm). Therefore, Q390B has a minimum yield strength of ≥ 390 MPa.
B: Represents the quality grade, which guarantees a specific impact toughness at +20°C.
2. Key Mechanical Properties
Properties are thickness-dependent. The following are for the most common thickness range:
Yield Strength (ReH):
≥ 390 MPa (for thickness t ≤ 16 mm)
Strength decreases for thicker plates (e.g., ≥ 370 MPa for 16 < t ≤ 40 mm, per standard tables).
Tensile Strength (Rm): 490 - 650 MPa
Elongation at Break (A): ≥ 19% (for t ≤ 40mm, longitudinal specimen)
Impact Toughness: Guaranteed Charpy V-notch impact energy ≥ 34 J at +20°C (transverse specimen). This is the key feature of the "B" grade.
3. Chemical Composition
Q390B achieves its higher strength through a controlled low-alloy composition with slightly higher levels of strengthening elements like Manganese (Mn) compared to Q355B, and may include microalloying elements like Vanadium (V) or Niobium (Nb). Key limits (typical):
Carbon (C): ≤ 0.20%
Silicon (Si): ≤ 0.55%
Manganese (Mn): ≤ 1.70%
Phosphorus (P) & Sulfur (S): Low limits (e.g., P≤0.030%, S≤0.030%) to ensure weldability and toughness.
Carbon Equivalent (Ceq): Typically around 0.45% or higher, making its weldability more demanding than Q355B.
4. Primary Applications
Q390B is used in heavily loaded structures or weight-sensitive designs where Q355B is insufficient, but the full strength of Q460 is not required. Typical uses include:
Heavy-duty industrial buildings (e.g., power plants, large workshops with very heavy cranes).
Large-span structures (stadium roofs, exhibition halls).
Key components of bridges (main girders, arches).
Mining and heavy machinery (frames, booms).
High-rise building cores in lower floors of very tall structures.
5. Comparison with Common Steels
| Steel Grade | Min Yield Strength (t≤16mm) | Key Difference vs. Q390B |
|---|---|---|
| Q355B | 355 MPa | Lower strength, better weldability. Q390B is its higher-strength upgrade. |
| Q390B | 390 MPa | Baseline high-strength grade. Stronger than Q355B, requires more care in welding. |
| Q420B | 420 MPa | Next strength level. Often used in similar but more demanding applications. |
| ASTM A572 Gr. 55 | 380 MPa (55 ksi) | American near-equivalent. Very close in strength and application. |
6. Important Processing Considerations
Welding: Due to its higher strength and carbon equivalent, welding Q390B requires strict procedure qualification. Low-hydrogen welding processes and often preheating are mandatory to prevent cold cracking. Post-weld heat treatment may be needed for thick sections.
Forming & Cutting: Can be cold formed and cut using standard methods (plasma, flame), but higher strength requires more force.
Summary
Q390B is a standardized Chinese high-strength structural steel with a minimum yield strength of 390 MPa and guaranteed impact toughness at room temperature. It is an economically efficient material for engineers to reduce weight and increase load capacity in structures where the common Q355B is inadequate, but it demands more careful fabrication, particularly in welding. Its use is governed by structural design codes like GB 50017.