Q690D and Q690E are both high-strength low-alloy (HSLA) structural steels compliant with GB/T 1591-2018 and GB/T 16270-2019 standards, sharing a minimum yield strength of 690 MPa for plates ≤50 mm. Their core distinction lies in low-temperature impact toughness requirements, which dictates their respective application scenarios, chemical composition control, and production processes.
Core Difference
The suffixes "D" and "E" represent different quality grades, with the key gap in impact test temperature and toughness thresholds-the most critical factor for material selection in cold environments.
| Index | Q690D | Q690E |
|---|---|---|
| Impact Test Temperature | -20℃ | -40℃ |
| Minimum Charpy V-notch Impact Energy | ≥47 J (average of 3 specimens) | ≥27 J (average of 3 specimens; actual engineering values often exceed 47 J) |
| Toughness Feature | Resists brittle fracture in moderately cold environments | Maintains stable performance in ultra-low-temperature conditions |
Chemical Composition
Both grades adopt a low-carbon, microalloyed design, but Q690E imposes stricter limits on harmful elements and optimizes alloy ratios to enhance ultra-low-temperature toughness.
Q690D
Carbon (C) ≤ 0.18%, manganese (Mn) ≤ 2.00%
Harmful elements: Phosphorus (P) ≤ 0.030%, sulfur (S) ≤ 0.025%
Microalloying elements: Niobium (Nb), vanadium (V), titanium (Ti) for grain refinement; no additional alloy optimization for ultra-low temperatures
Carbon equivalent (Ceq) ≤ 0.55%, ensuring basic weldability
Q690E
Carbon (C) ≤ 0.18%, with more precise control (some batches ≤ 0.15%) to improve ductility
Harmful elements: Phosphorus (P) ≤ 0.025%, sulfur (S) ≤ 0.020%-stricter limits to avoid embrittlement at -40℃
Microalloying elements: Adds appropriate chromium (Cr) and nickel (Ni) to enhance low-temperature toughness; Nb/V/Ti content is optimized for finer grain structure
Carbon equivalent (Ceq) ≤ 0.47%, with better weldability for thick plates
Production Process
Both grades can use thermo-mechanical control process (TMCP) or quenching & tempering (Q&T) technology, but Q690E requires more precise process parameters to guarantee ultra-low-temperature performance.
- Q690D
- For medium-thin plates: TMCP (controlled rolling + accelerated cooling) to form fine-grained ferrite-bainite structure
- For thick plates (>50 mm): Q&T process (quenching at 880–920℃, tempering at 550–600℃) to ensure toughness at -20℃
- Routine non-destructive testing (ultrasonic testing for internal defects)
- Q690E
Adopts enhanced TMCP: Higher cooling rate after rolling to obtain finer grains; some products need secondary tempering to eliminate residual stress
For critical components: Adds normalizing treatment at 900–950℃ to homogenize microstructure
Strict quality inspection: 100% ultrasonic testing + surface magnetic particle testing; batch sampling for -40℃ impact tests to ensure consistency
Application Scenarios
- The selection of Q690D or Q690E is mainly determined by the minimum service temperature of the project.
- Q690D Application Scope
- Suitable for regions with minimum temperatures above -20℃, such as northern China, central Asia, and parts of Europe
- Typical uses: Crane booms, wind turbine tower flanges, large-span bridge truss components, coal mine hydraulic supports, heavy truck frames
- Q690E Application Scope
Suitable for extreme cold regions with temperatures as low as -40℃, such as high-latitude areas (northeast China, Siberia, Canada), deep-sea engineering, and polar equipment
Typical uses: Alpine section pipelines of natural gas projects, polar LNG storage tank supports, deep-sea drilling platform jackets, low-temperature pressure vessels
Cost and Welding Requirements
| Aspect | Q690D | Q690E |
|---|---|---|
| Cost | Lower; 10–20% cheaper than Q690E | Higher; increased costs from stricter composition control, precise processes, and rigorous testing |
| Welding Preheating Temperature | 100–150℃ for plates ≥12 mm | 120–180℃ for plates ≥12 mm; higher preheating to prevent hydrogen-induced cracking |
| Post-weld Treatment | Hydrogen removal treatment optional for thick plates | Mandatory hydrogen removal treatment at 550–600℃ for all welded components |
What is the most essential difference between Q690D and Q690E, and how does it affect material selection?
The most essential difference lies in low-temperature impact toughness requirements. Q690D must pass the Charpy V-notch impact test at -20℃ with an average impact energy of ≥34 J, while Q690E needs to meet the test standard at -40℃ with an average impact energy of ≥27 J (actual values often exceed 47 J in engineering). This difference directly determines the selection: Q690D is suitable for moderately cold areas where the minimum temperature is above -20℃, while Q690E is designed for extreme cold environments with temperatures as low as -40℃, such as high-latitude regions and deep-sea projects.
Are there obvious differences in the welding processes of Q690D and Q690E?
Yes, there are targeted differences mainly due to Q690E's stricter low-temperature performance requirements. For Q690D, plates ≤12mm can be welded without preheating; for thicker plates (≥12mm), the preheating temperature is controlled at 100–150℃, and post-weld hydrogen removal treatment is optional. For Q690E, regardless of plate thickness, low-hydrogen welding materials are mandatory, the preheating temperature is higher (120–180℃ for plates ≥12mm), and post-weld hydrogen removal treatment at 550–600℃ is required to prevent weld joint embrittlement in ultra-low-temperature environments.
Why does Q690E cost more than Q690D, and what factors contribute to the price gap?
Q690E is 10–20% more expensive than Q690D, and the price gap comes from three aspects. First, raw material costs: Q690E has stricter limits on harmful elements (P≤0.025%, S≤0.020%) and adds optimized alloy elements such as chromium and nickel to enhance low-temperature toughness. Second, production processes: Q690E requires additional VD vacuum degassing during smelting and more precise TMCP parameters during rolling, which increases process complexity and energy consumption. Third, quality inspection costs: Q690E needs 100% ultrasonic testing and batch - level -40℃ impact tests, while Q690D only requires routine inspections.