Q690E is a premium grade of high-strength low-alloy (HSLA) structural steel compliant with Chinese national standards, primarily GB/T 1591-2018 and GB/T 16270-2009. Renowned for its exceptional strength-toughness balance, excellent weldability, and superior low-temperature performance, it serves as a core material for heavy-duty engineering applications in extreme environments.
China GB/T16270 Q690E Carbon and Low-alloy High-strength Steel Plate Chemical Composition and Mechanical Property
| STEEL GRADE | C% max | Si% max | Mn% max | P% max | S% max | Cu% max | Cr% max | Ni% max | Mo % max | B % max | V % max | Nb% max | Ti % max |
| Q690E | 0.2 | 0.80 | 1.8 | 0.025 | 0.015 | 0.5 | 1.5 | 2 | 0.70 | 0.005 | 0.12 | 0.06 | 0.05 |
| STEEL GRADE | Re yield strength Mpa min | Re yield strength Mpa min | elongation % | Energy and temperature | |||||||
| Min | 0 | -20 | -40 | -60 | |||||||
| t≤50 | 50-100 | 100-150 | t≤50 | 50-100 | 100-150 | J | J | J | J | ||
| Q690E | 690 | 650 | 630 | 770-940 | 760-930 | -- | 14 | 47 | 47 | 34 | 34 |
Grade Nomenclature
The designation of Q690E carries clear technical implications:
- Q: Abbreviation for "yield strength" in Chinese pinyin, indicating the steel's classification based on yield strength performance.
- 690: Minimum yield strength of 690 MPa for steel plates with thickness ≤16 mm. This value decreases moderately with increasing thickness (e.g., ≥650 MPa for 50＜t≤100 mm).
- E: Quality grade symbol, requiring the steel to pass a -40℃ Charpy V-notch impact test with a minimum impact energy of 27 J. This ensures resistance to brittle fracture in ultra-low-temperature conditions.
Mechanical Properties
- Tensile strength: 770–940 MPa (for t≤50 mm), with elongation ≥14%, ensuring good plastic deformation capacity while bearing heavy loads.
- Low-temperature toughness: Maintains stable impact performance at -40℃, far exceeding the requirements of lower quality grades (e.g., D grade for -20℃).
- Hardness: Typically ranges from HB 241–353, balancing wear resistance and processability.
Chemical Composition
Q690E adopts a low-carbon, microalloyed design to optimize strength and toughness:
- Carbon (C): ≤0.18%, reducing weld cracking risk and improving low-temperature ductility.
- Key alloy elements: Manganese (Mn ≤2.00%), chromium (Cr), and nickel (Ni) enhance hardenability; niobium (Nb), vanadium (V), and titanium (Ti) refine grains for better strength-toughness synergy.
- Harmful element control: Phosphorus (P ≤0.025%) and sulfur (S ≤0.020%) are strictly limited to avoid embrittlement.
- Carbon equivalent (Ceq): ≤0.47%, ensuring excellent weldability without complex preheating procedures for thin plates.
What is the core advantage of Q690E steel compared to lower-grade high-strength steels like Q550D or Q620E?
The core advantage of Q690E lies in its superior strength - toughness balance, especially in ultra-low-temperature environments. With a minimum yield strength of 690MPa (for thickness ≤16mm), it delivers higher load-bearing capacity than Q550D (550MPa yield strength) and Q620E (620MPa yield strength), enabling significant weight reduction of structural components-typically 10–20% lighter than Q620E for the same load requirements. Meanwhile, it retains excellent impact toughness at -40℃, which avoids the trade-off between strength and toughness that often occurs in lower-grade steels, making it ideal for heavy-duty, low-temperature applications.
What kind of heat treatment processes are usually applied to Q690E steel, and what are their purposes?
Q690E steel commonly adopts two main heat treatment routes, depending on the application requirements. First is the thermo-mechanical control process (TMCP), which combines controlled rolling and accelerated cooling. This process refines the grain structure into fine-grained ferrite and bainite, enhancing both strength and toughness without the need for additional quenching and tempering. Second is the quenching and tempering (Q&T) process for thick plates (＞50mm): the steel is heated to 900–950℃ for quenching to form martensite, then tempered at 550–650℃ to reduce internal stress and improve ductility. Both processes aim to optimize the steel's microstructure, ensuring it meets the strict strength and low-temperature toughness criteria.
Can Q690E steel be used in marine or offshore engineering projects? What key considerations are there?
: Yes, Q690E is well-suited for marine and offshore engineering, such as deep-sea drilling platform jackets, offshore wind turbine support structures, and LNG ship hull components. The key considerations are corrosion resistance and welding reliability. Since marine environments are highly corrosive, Q690E is often paired with protective measures like hot-dip galvanizing or heavy-duty anti-corrosion coatings. For welding, low-hydrogen welding consumables must be used, with strict control over preheating temperature (120–180℃ for plates ≥20mm) and interpass temperature (≤200℃) to prevent hydrogen-induced cracking and ensure weld joint performance matches the base metal.