Q620E and Q690E are both high-strength low-alloy structural steels of grade E in China. They can maintain good impact toughness at -40℃ and are mainly delivered in quenched and tempered state. However, there is a 70MPa gap in their yield strength.
Chemical Composition
Both adopt low-carbon design to balance strength and toughness, but Q690E adds more alloy elements such as Mo and B to enhance hardenability, and the content of harmful elements is controlled more strictly. The specific comparison is as follows:
| Element | Q620E | Q690E |
|---|---|---|
| Carbon (C) | ≤0.18 (some standards allow ≤0.20) | ≤0.18 - 0.20 |
| Silicon (Si) | ≤0.50 | ≤0.60 - 0.80 |
| Manganese (Mn) | ≤1.50 | ≤1.70 - 2.00 |
| Phosphorus (P) | ≤0.035 | ≤0.025 - 0.030 |
| Sulfur (S) | ≤0.035 | ≤0.015 - 0.025 |
| Alloy elements | Mainly Nb, V, Ti, which refine grains through precipitation strengthening | Contains Ni≤0.80, Cr≤1.00, Mo≤0.30, B≤0.004, etc., which further improve strength and hardenability while ensuring toughness |
Mechanical Properties
The core difference lies in the yield strength and low-temperature impact toughness. Q690E has higher strength and stricter impact toughness requirements, while Q620E has slightly better plasticity in terms of elongation in some standards. The specific indicators are as follows (for thickness ≤50mm):
| Performance Indicator | Q620E | Q690E |
|---|---|---|
| Yield strength | ≥620MPa (some standards require ≥630MPa) | ≥690MPa |
| Tensile strength | 710 - 880MPa | 770 - 940MPa |
| Elongation | ≥14% (some can reach ≥16%) | ≥14% |
| Impact toughness at -40℃ | ≥27J (partial standard requires ≥34J) | ≥47J, and the single sample value shall not be less than 70% of the average value |
| Section shrinkage rate | ≥45% | No clear uniform standard, but it is generally required to match the toughness to ensure the deformation capacity |
Differences in Production Process and Processing Difficulty
Both are mainly delivered by quenching and tempering or TMCP process, but Q690E has higher requirements for process precision and process control to ensure the stability of ultra-high strength, and the processing difficulty is significantly higher.
- Q620E: Balanced process, easy to popularize. It can be produced by either quenching and tempering or TMCP process. When using the TMCP process, the final rolling temperature is controlled at about 850 - 880℃, and the cooling rate is 5 - 8℃/s. The mild process parameters can avoid excessive internal stress. Its carbon equivalent is ≤0.48%, the welding preheating temperature is 150 - 200℃, and ordinary gas shielded welding can meet the requirements. After forming, only simple annealing is needed to solve the problem of stress concentration, which is suitable for large-scale batch production.
- Q690E: Precision process, high control requirements. It often relies on the combination of micro-alloying and strict quenching and tempering process. In the smelting stage, spectral analysis is used to monitor the content of alloy elements in real time. The quenching temperature is accurately controlled at 880 - 920℃, and the tempering temperature is 580 - 650℃ to obtain fine-grained structure. During welding, the preheating temperature needs to be controlled above 100 - 150℃, and low-hydrogen welding materials must be used to avoid cold cracks. When processing special-shaped components such as crane booms, the deformation force and cooling rate must be precisely controlled to prevent performance degradation caused by uneven structure.
Differences in Application Scenarios
The two steels are divided according to the stress-bearing requirements of components. Q620E is oriented to medium and high stress components, and Q690E is more used for ultra-high stress core components, and the latter has more advantages in lightweight and extreme working conditions.
- Q620E: Cost-effective choice for medium and high stress fields. In engineering machinery, it is used to make 400-ton crane booms, which can reduce weight by 18% compared with traditional materials. In the energy field, it is applied to the 7MPa high-pressure pipe section of the Xinjiang coal-to-gas transmission pipeline, which reduces the wall thickness by 10% and saves about 80 tons of steel per kilometer. In the construction field, it is used for the truss nodes of 300-meter super high-rise buildings, and its cyclic load performance is 40% higher than that of traditional materials. It also performs well in offshore wind power monopile transition sections, and its service life can reach 35 years with anti-corrosion protection.
- Q690E: Core material for ultra-high stress and extreme conditions. In energy exploitation, it is used for pressure-bearing pipe fittings of deep-sea drilling platforms and fracturing pipelines for shale gas exploitation, and its service life is 3 times longer than that of conventional materials. In engineering projects, the pressure steel pipe of Baihetan Hydropower Station adopts Q690E, which reduces the wall thickness from 60mm to 42mm and saves 12,000 tons of steel. In heavy machinery, it is used to make the frame of large excavators. The service life of the bucket made of Q690E is 30% longer than that of ordinary steel. It is also used for high-speed rail bogie components, which can reduce weight by 15% while ensuring anti-fatigue performance.
Differences in Market and Cost
The differences in process complexity and raw material consumption lead to obvious gaps in the market supply and demand pattern and cost of the two steels:
- Q620E: It has a mature production process, and many medium and large steel mills can stably produce it. The market supply is sufficient, and the price is relatively moderate. It is widely used in general high-strength projects, with stable market demand and an annual growth rate of about 10%. Its cost advantage is obvious, and it is often the first choice for projects with balanced performance and cost requirements.
- Q690E: The cost of alloy elements such as Mo and Ni is high, and the precision control of the production process also increases the energy consumption cost. Its market price is about 20% - 30% higher than that of Q620E. It is mainly produced by key large steel mills. The demand is concentrated in high-end fields such as deep-sea energy development and large-scale hydropower projects. It is gradually replacing imported materials, and the cost can be reduced by more than 30% compared with imported ASTMA519 4130 materials.
How to choose between Q620E and Q690E in the construction of long-span bridges in alpine regions?
If it is an ordinary long-span bridge with no excessive requirements for weight reduction, Q620E is more suitable. It can meet the bearing needs and reduce the procurement and construction costs. If the bridge is located in an extremely cold area with frequent strong winds, and needs to reduce the structural weight to improve seismic and wind resistance, Q690E is preferred. Its higher strength and impact toughness of ≥47J at -40℃ can effectively avoid brittle fracture, and the thinning of components can also reduce the overall wind load.
What technical adjustments are needed when replacing Q620E with Q690E in the transformation of hydraulic supports in coal mines?
First, adjust the welding process. Use low-hydrogen high-strength welding materials, raise the preheating temperature to 150℃ or above, and reduce the welding heat input to prevent the softening of the heat-affected zone. Second, optimize the heat treatment process after processing. Temper at 580 - 650℃ to eliminate internal stress. Finally, adjust the forming parameters, slow down the bending and stamping speed, and avoid cracks caused by the high strength and relatively low plasticity of Q690E.
Why is Q690E more favored in the field of large-scale engineering machinery lightweight?
Under the condition of bearing the same load, Q690E can greatly reduce the thickness and volume of components due to its 70MPa higher yield strength than Q620E. For example, after using Q690E for the arm frame of a certain type of crane, the maximum lifting weight is increased from 800 tons to 1200 tons while reducing weight. This lightweight advantage can not only improve the equipment's load capacity but also reduce energy consumption during operation, which is in line with the development trend of high-efficiency and energy-saving engineering machinery.