Q620D and Q690D are two representative D-grade high-strength structural steels, both satisfying impact toughness requirements at minus 20 degrees Celsius. The 70 MPa gap in yield strength is not just a numerical difference but reflects different design orientations, engineering application boundaries, and cost-benefit trade-offs. Q620D follows a balanced design that controls cost and keeps the carbon equivalent low for easy welding and forming, while Q690D is precision-engineered to break through the 690 MPa threshold with strict impurity control and, for thick plates, a quenching and tempering route. This article compares the two grades across design logic, engineering adaptability, processing requirements, and cost-benefit ratio to support material selection.
Design Logic
The core difference between Q620D and Q690D lies in their design starting points. Q620D is positioned as a cost-effective high-strength steel whose design logic is to balance strength, processability, and production cost. It adopts a low-carbon formula with a maximum of 0.20 percent carbon and relies on the synergistic effect of conventional microalloy elements such as niobium, vanadium, and titanium to achieve strength through grain refinement and precipitation strengthening, without expensive elements such as molybdenum and nickel. Q690D is designed to break through the 690 MPa yield threshold, with its core goal being ultra-high strength while maintaining low-temperature toughness, which requires precise control of microalloy proportions and harmful impurities.
Engineering Adaptability
Q620D is widely used in projects that require high strength but do not demand extreme load-bearing capacity. In engineering machinery, it serves chassis of medium-tonnage loaders, frames of small cranes, and connecting parts of concrete pump trucks, with processing cost about 15 to 20 percent lower than Q690D. In building structures, it is applied to load-bearing trusses of large-span workshops, auxiliary supports of high-rise buildings, and piers of urban overpasses, where it avoids brittle fracture at minus 20 degrees Celsius. In energy equipment, it is used for support structures of land wind power towers and low-pressure pipeline sections, with service life reaching about 25 years. Q690D targets key components under ultra-high loads, such as coal mine hydraulic supports, where replacing Q620D raises working resistance from 8000 kN to 12000 kN and reduces column weight by about 18 percent.
Processing Matching Degree
The two grades differ clearly in processing requirements. Q620D has a low carbon equivalent of at most 0.45 percent, so thin plates up to 20 mm need no preheating, thick plates of 30 mm and above need preheat of only 100 to 150 degrees Celsius, and ordinary gas-shielded consumables such as ER50-6 are sufficient. Flame cutting applies to all thicknesses, cold bending is possible for plates up to 30 mm without preheating, and the minimum bending radius is 3 to 4 times the plate thickness. Q690D requires low-hydrogen consumables, preheat of 150 to 200 degrees Celsius for plates of 20 mm and above, and heat input controlled at 15 to 25 kJ per centimeter to prevent heat-affected zone softening. Plasma or laser cutting is recommended, cold bending needs a larger radius of at least 5 to 6 times the thickness, and post-weld hydrogen removal treatment is mandatory for key components.
Cost-Benefit Ratio
Q620D has a market price of roughly 6000 to 7500 yuan per tonne, with production cost dominated by raw material and rolling costs because alloy additions are low and processing can use ordinary equipment. Q690D typically sells at 7200 to 9000 yuan per tonne, about 15 to 25 percent higher, because of higher alloy cost, heat treatment, and precision inspection. In long-term value, Q620D suits general projects with a service life of 20 to 25 years, while Q690D suits key projects with a service life of 30 to 35 years and lower maintenance frequency. For large-scale projects, a mixed strategy can be used: Q690D for core load-bearing components and Q620D for auxiliary structural parts.
Selection Guidelines and Notes
Selection should follow the load-bearing level and working environment of the component. For non-key structural parts and general heavy-load scenarios, choose Q620D; for core load-bearing components and harsh low-temperature environments, choose Q690D. When replacing Q620D with Q690D, the welding process must be adjusted: use low-hydrogen materials, increase preheat, control heat input, and perform post-weld hydrogen removal treatment for key components. When replacing Q690D with Q620D, the substitution is only applicable to non-key structural parts and must be verified by structural strength calculation to avoid safety risks from insufficient strength.
Frequently Asked Questions
What is the core difference between Q620D and Q690D? The core difference is design positioning and yield strength: Q620D is a cost-effective balanced grade, while Q690D is precision-engineered for ultra-high strength with stricter impurity control.
What is the carbon equivalent of Q620D? The carbon equivalent of Q620D is at most about 0.45 percent, which keeps the steel easy to weld and form without complex preheating.
What are the welding requirements for Q690D? Q690D requires low-hydrogen consumables, preheat of 150 to 200 degrees Celsius for plates of 20 mm and above, heat input controlled at 15 to 25 kJ per centimeter, and post-weld hydrogen removal treatment for key components.
Where is Q620D commonly used? Q620D is used for loader chassis, small crane frames, workshop trusses, high-rise auxiliary supports, overpass piers, wind tower supports, and low-pressure pipeline sections.
Where is Q690D commonly used? Q690D is used for coal mine hydraulic supports, port crane booms, large pump truck arms, mining dump truck chassis, and bridge and high-rise components in frigid regions.
How do the prices compare? Q690D is about 15 to 25 percent more expensive than Q620D, reflecting higher alloy cost, heat treatment, and precision inspection requirements.