Q620D and Q690D are both D - grade low-alloy high-strength structural steels in China. They both need to meet the impact toughness requirement at -20℃, which is widely used in machinery manufacturing and engineering construction. However, there is a 70MPa gap in their yield strength, which leads to differences in chemical composition, processing technology, application scenarios and cost.
Both steels adopt low-carbon and micro-alloying design, but Q690D has a more refined element ratio and stricter control of harmful impurities to achieve higher strength while ensuring toughness. The specific comparison is as follows:
| Element | Q620D | Q690D |
|---|---|---|
| Carbon (C) | ≤0.20% (some standards require ≤0.18%) | ≤0.20% (controlled at a low level to avoid toughness reduction) |
| Silicon (Si) | ≤0.60% | ≤0.60% |
| Manganese (Mn) | ≤1.80% | ≤1.80% (enhances hardenability and strength through solid solution strengthening) |
| Phosphorus (P) | ≤0.035% (high - end products can be ≤0.020%) | ≤0.030% |
| Sulfur (S) | ≤0.035% (high - end products can be ≤0.010%) | ≤0.020% (some high - end products can be ≤0.015%) |
| Alloy elements | Mainly add Nb, V, Ti, and improve strength through grain refinement and precipitation strengthening | On the basis of Nb, V, Ti, the content of micro - alloy elements is adjusted. For example, Nb≤0.06%, V≤0.12%, Ti≤0.20%, which strengthens the synergistic effect of grain refinement and precipitation strengthening |
Obvious Gap in Mechanical Properties
The core difference between the two steels lies in the yield strength. Meanwhile, there are also differences in tensile strength and low-temperature impact toughness, which determine their different load-bearing capacities:
| Mechanical Property Indicator | Q620D | Q690D |
|---|---|---|
| Yield strength | ≥620MPa (for thickness ≤50mm) | ≥690MPa (for thickness ≤50mm, slightly fluctuates with the increase of plate thickness) |
| Tensile strength | 710 - 880MPa | 770 - 940MPa |
| Elongation | ≥15% (some products can reach ≥16%) | ≥14% (some customized products can reach 17%) |
| Impact toughness at -20℃ | Impact energy ≥34J, and some high - standard products can reach ≥55J | Impact energy ≥34J, and some high - end products can achieve impact performance at -40℃ |
Differences in Processing Technology
The two steels can be produced by controlled rolling and controlled cooling process, but Q690D has higher requirements for process control to ensure the stability of ultra-high strength, and the processing difficulty is slightly higher.
- Q620D: Its production process is relatively balanced and easy to promote. It can be stably produced by the controlled rolling and controlled cooling (TMCP) process. Its carbon equivalent is ≤0.45%, and there is no need for complex preheating during welding. Ordinary gas shielded welding can meet the requirements. For plates ≤30mm, cold bending can be directly carried out without preheating, which is suitable for large - batch production and can effectively reduce the production cycle.
- Q690D: It needs more strict process control in the smelting stage. It usually adopts converter or electric arc furnace smelting, combined with LF and VD furnace external refining processes to improve the purity of molten steel. For thick plates, normalizing or quenching and tempering heat treatment may also be required to optimize the structure. When welding, low - hydrogen welding materials should be selected. For thick plates, it is necessary to properly control the heat input to avoid the decline of toughness caused by coarse grains in the heat - affected zone, and post - welding stress relief heat treatment can be carried out if necessary.
Differences in Application Scenarios
The difference in strength makes the two steels have distinct positioning in application scenarios. Q620D is oriented to medium - and high - load components, while Q690D is more used for core load-bearing components, and has more advantages in cold regions.
- Q620D: It is a cost-effective choice in the field of medium - and high - strength engineering. In the field of engineering machinery, it is used to manufacture the chassis of medium - tonnage loaders and the frames of small and medium - sized cranes. In the field of building structures, it is applied to the load-bearing components of general large - span workshops and the connecting parts of wind power towers on land. In the field of chemical industry, it can be used to make medium - pressure pipeline supports, which can reduce the wall thickness of components compared with traditional steel and save steel consumption.
- Q690D: It is a core material for heavy - duty and low - temperature resistant components. In the field of coal mine machinery, it replaces Q550 - grade materials for coal mine hydraulic supports, increasing the working resistance of the column from 8000kN to 12000kN. In the field of engineering machinery, it is used for the boom of 56 - meter pump trucks and port cranes, which can reduce the weight of components by 23%. In the field of construction in northern China, it is used for the seismic supports of super high - rise buildings and the key parts of bridges, and its low - temperature toughness can effectively avoid brittle fracture in cold winter.
Differences in Market and Cost
The differences in process difficulty and element configuration make the market prices and supply patterns of the two steels quite different.
- Q620D: Its production process is mature, and many medium - sized steel mills can realize mass production. The market supply is sufficient. Its price is relatively moderate because it does not need to add a large number of alloy elements and the process control cost is low. It is widely used in general high - strength projects, and the market demand is stable, which is suitable for projects with limited cost and general performance requirements.
- Q690D: The cost of smelting and refining is high due to the strict control of harmful impurities and the optimization of micro - alloy element ratio. Its market price is about 15% - 25% higher than that of Q620D. It is mainly produced by key large - scale steel mills. Its demand is concentrated in high - end fields such as coal mine hydraulic supports and large - tonnage engineering machinery. With the continuous maturity of domestic production technology, it is gradually replacing imported similar materials, and the cost advantage compared with imported materials is more obvious.
Key Notes for Material Selection
- For general low - temperature and medium - load projects: Such as the frame of small cranes and the load-bearing structure of ordinary workshops in northern cities, Q620D is preferred. It can meet the basic low - temperature and load-bearing requirements, and the lower procurement and construction costs can effectively control the overall project budget.
- For heavy - load and harsh low - temperature projects: Such as coal mine hydraulic supports and the boom of large pump trucks, Q690D should be selected. Its ultra - high strength can reduce the weight of components while improving the load - bearing capacity. At the same time, its stable low - temperature toughness can avoid safety risks caused by harsh working conditions.
- When replacing materials: If Q620D is to be replaced with Q690D, the welding process should be adjusted, such as using low - hydrogen welding materials and controlling the welding heat input. If Q690D is replaced with Q620D, it is necessary to verify whether the structural strength meets the requirements, and it is not recommended to replace it in key load-bearing components.
What is the core difference in mechanical properties between Q620D and Q690D, and how does it affect their application scope?
The core difference lies in yield strength. Q620D has a minimum yield strength of ≥620MPa, while Q690D reaches ≥690MPa (for thickness ≤50mm). Q690D also has a slightly higher tensile strength (770-940MPa vs 710-880MPa for Q620D). This strength gap directly defines their application scope: Q620D is suitable for general heavy-load components with moderate strength requirements, while Q690D is designed for core load-bearing parts that need to withstand ultra-high loads and harsh low-temperature environments.
What are the key differences in welding requirements between Q620D and Q690D during on-site construction?
The welding requirements differ significantly due to their carbon equivalent and strength levels. For Q620D, with a carbon equivalent ≤0.45%, thin plates (≤20mm) can be welded without preheating, and ordinary gas-shielded welding materials (e.g., ER50-6) are sufficient; preheating temperature for thick plates (≥30mm) only needs to be 100-150℃, and no post-weld heat treatment is required for general components. For Q690D, low-hydrogen welding materials must be used to prevent cold cracks; preheating temperature for thick plates needs to be increased to 150-200℃, and welding heat input must be strictly controlled within 15-25kJ/cm. In addition, post-weld hydrogen removal heat treatment is mandatory for key load-bearing components to ensure performance stability.
When building a large-span workshop in a northern China area with minimum temperatures of -20℃, how to choose between Q620D and Q690D?
The choice depends on the workshop's load-bearing requirements and budget. If it is an ordinary workshop with a span ≤30m and no heavy equipment (e.g., small crane systems), Q620D is more cost-effective-it can meet the low-temperature impact toughness requirement and control the overall project cost. If the workshop has a span ≥40m or needs to bear heavy equipment (e.g., large-tonnage gantry cranes), Q690D is the better option. Its higher yield strength can reduce the cross-sectional area of truss components, save floor space, and improve structural stability under low-temperature alternating stress.