Q960D and Q690D are both domestic low-alloy high-strength structural steels with the same quality grade "D" (meeting the impact toughness requirement at -20℃), but there are obvious gaps in strength levels. These gaps further lead to differences in chemical composition, processing requirements, application scenarios and other aspects.
Chemical Composition
Both steels strictly control harmful impurities to ensure toughness, but Q960D has stricter control over phosphorus and sulfur content to match its ultra-high strength, and the proportion of alloy elements is more optimized.
| Element | Q960D | Q690D |
|---|---|---|
| Carbon (C) | ≤0.20% | 0.18%-0.28% |
| Silicon (Si) | ≤0.80% | 0.20%-0.80% |
| Manganese (Mn) | ≤2.00% | 1.20%-1.80% |
| Phosphorus (P) | ≤0.025% | ≤0.025% |
| Sulfur (S) | ≤0.015% | ≤0.015% |
| Alloy elements | Contains Cr (≤1.50%), Ni (≤2.00%), Mo (≤0.70%) and other elements to enhance strength, and adds Nb, Ti and other elements to refine grains | Contains Cr (0.30%-0.60%), Mo (0.20%-0.40%) and other elements, and micro-alloying elements such as Nb and Ti are added to balance strength and toughness |
Mechanical Properties
The most core difference between the two lies in the yield strength. Q960D belongs to ultra-high-strength steel, while Q690D is high-strength steel. The specific performance indicators are quite different:
| Performance Indicator | Q960D | Q690D |
|---|---|---|
| Yield strength | ≥960MPa (for thickness ≤50mm) | ≥690MPa (for thickness ≤16mm; ≥630MPa when thickness increases to 100mm) |
| Tensile strength | 980 - 1150MPa | 730 - 900MPa |
| Elongation | ≥10% | ≥16% |
| Impact toughness | Meets the impact test requirement at -20℃, with excellent resistance to brittle fracture | Impact energy ≥34J at -20℃, and outstanding low-temperature anti-fracture ability |
| Fatigue performance | Higher yield strength leads to lower fatigue crack growth rate, and better resistance to alternating load | Good anti-fatigue performance, which can meet the requirements of general dynamic load scenarios (fatigue life ≥2×10⁶ times according to ISO 12107 standard) |
Processing and Welding Requirements
The differences in strength and structure make their processing difficulty and process parameters distinct:
- Q960D: It is delivered in the quenched and tempered state. For thick plates above 30mm, preheating is required before flame cutting to avoid hardening of the heat-affected zone. When welding, the carbon equivalent needs to be strictly controlled. Although its composition design considers weldability, higher welding preheating temperature and more precise heat input control are still required to prevent welding cracks.
- Q690D: It can be delivered in multiple states such as hot rolling, normalizing and TMCP. Its welding performance is more balanced, with a carbon equivalent ≤0.65%. It is suitable for arc welding, gas shielded welding and other processes. The welding process is relatively conventional, and the qualified rate of welds is over 98%.
Application Scenarios
The application fields are differentiated according to load-bearing requirements and cost considerations:
- Q960D: It is mainly used in key load-bearing components with high strength requirements. For example, it is used for coal mine hydraulic supports (replacing Q690 steel can reduce the weight of a single hydraulic support by 20%), wind power towers, port cranes, and pressure-resistant shells of deep submersibles. It is also applied to the frame of mining trucks and other heavy equipment that requires both high strength and weight reduction.
- Q690D: It is widely used in general high-load scenarios. In the field of infrastructure, it is used for beams and columns of super high-rise buildings and main load-bearing structures of large-span bridges. In the field of equipment manufacturing, it is used for the chassis of heavy vehicles, ship decks, and components of construction machinery such as excavators. It can also be used for pressure vessels and oil and gas pipelines in the energy field.
What is the core difference in mechanical properties between Q960D and Q690D?
The core difference lies in the yield strength. Q960D is an ultra-high-strength steel with a minimum yield strength of 960MPa and a tensile strength ranging from 980MPa to 1150MPa, but its elongation is only ≥10%. Q690D is a high-strength steel with a minimum yield strength of 690MPa (for thickness ≤16mm) and a tensile strength of 730MPa - 900MPa. It has better ductility with an elongation of ≥16%. In addition, Q960D has a lower fatigue crack growth rate and stronger resistance to alternating loads.
Do Q960D and Q690D have differences in their delivery states and processing difficulty?
Yes, there are obvious differences. Q960D is mainly delivered in the quenched and tempered state to ensure its ultra-high strength and structural uniformity. For thick plates, preheating is required during cutting, and strict control of heat input is necessary during welding to avoid cracks. Q690D has more flexible delivery states, including hot rolling, normalizing, TMCP, etc. Its cutting and welding processes are more conventional, and it does not require extremely strict process parameters, so the overall processing difficulty is lower than that of Q960D.
In the field of engineering machinery, when is it more appropriate to choose Q960D instead of Q690D?
Q960D is preferred when the equipment has strict requirements on weight reduction while bearing high loads. For example, when manufacturing coal mine hydraulic supports and port crane booms, using Q960D instead of Q690D can reduce the weight of the equipment by about 20% under the same load-bearing capacity, which significantly improves the equipment's energy efficiency and operational flexibility. However, if the equipment only needs to meet general high-load requirements and the budget is limited, Q690D is more cost-effective.
Are there differences in the application scope of Q960D and Q690D in the energy field?
Their application focuses are different. Q960D is mainly used for key high-pressure and high-strength components, such as high-pressure vessels in the energy field and core load-bearing parts of wind power towers. Q690D is more widely used in relatively conventional energy equipment, such as common wind power tower components, oil and gas pipelines, and low-temperature storage tanks. It can meet the strength and low-temperature toughness requirements of general energy equipment without excessive manufacturing costs.
Why does Q960D have stricter control over alloy elements than Q690D?
Because Q960D needs to achieve a much higher yield strength than Q690D. It not only controls the content of C, Si and Mn but also adds higher proportions of Cr, Ni and Mo to enhance the strength of the steel through solid solution strengthening and precipitation strengthening. At the same time, elements such as Nb and Ti are added to refine grains, which can make up for the reduction of toughness caused by the improvement of strength. Q690D only needs to balance general high strength and processability, so the type and proportion of alloy elements are relatively moderate.