Q620D and Q690D are both D-grade low-alloy high-strength structural steels in the Chinese GB/T 1591 series, and both must meet impact toughness requirements at minus 20 degrees Celsius. They are widely used in machinery manufacturing and engineering construction, but a 70 MPa gap in yield strength creates real differences in chemical composition, processing technology, application scenarios, and cost. Both steels use a low-carbon microalloying design, while Q690D applies a more refined element balance and stricter control of harmful impurities to reach higher strength without losing toughness. This article compares the two grades in detail.
Q620D and Q690D
The D grade designation means that the steel guarantees impact energy of at least 34 joules at minus 20 degrees Celsius, which suits components operating in cold conditions. Q620D provides a minimum yield strength of 620 MPa and Q690D a minimum of 690 MPa for thicknesses up to 50 mm. Below the 690 MPa level, Q620D offers a mature and economical route to high strength; above it, Q690D represents a more demanding product with tighter process control. The two grades therefore occupy adjacent steps on the strength ladder rather than competing directly in the same applications.
Chemical Composition Comparison
Both grades limit carbon to a maximum of 0.20 percent and silicon to 0.60 percent, with manganese up to 1.80 percent. The main difference is in impurity control: Q620D allows phosphorus up to 0.035 percent and sulfur up to 0.035 percent, while Q690D restricts phosphorus to 0.030 percent and sulfur to 0.020 percent. Both rely on niobium, vanadium, and titanium for grain refinement and precipitation strengthening. Q690D tunes the microalloy balance more tightly, with values such as niobium up to 0.06 percent, vanadium up to 0.12 percent, and titanium up to 0.20 percent, to strengthen the combined effect of grain refinement and precipitation.
Mechanical Property Comparison
For thicknesses up to 50 mm, Q620D has a minimum yield strength of 620 MPa and Q690D of 690 MPa. Tensile strength is 710 to 880 MPa for Q620D and 770 to 940 MPa for Q690D. Minimum elongation is 15 percent for Q620D, with some products reaching 16 percent, and 14 percent for Q690D, with some customized products reaching 17 percent. Both guarantee at least 34 joules of impact energy at minus 20 degrees Celsius, and some higher-standard Q690D products can achieve impact performance at minus 40 degrees Celsius, which matters for extreme cold-region service.
Processing and Welding Differences
Q620D can be produced stably by controlled rolling and controlled cooling, with a carbon equivalent of at most about 0.45 percent. Thin plates up to about 20 mm can be welded without preheating using ordinary gas-shielded consumables such as ER50-6, and plates up to 30 mm can be cold-bent directly, which suits large-batch production. Q690D demands stricter smelting with converter or electric arc furnace refining and ladle treatments to raise steel purity, and thick plates may need normalizing or quenching and tempering. Welding requires low-hydrogen consumables, preheat of 150 to 200 degrees Celsius for thick plates, heat input kept within 15 to 25 kJ per centimeter, and post-weld hydrogen removal treatment for key load-bearing components.
Applications, Cost and Selection
Q620D serves medium and high load components such as chassis of medium-tonnage loaders, frames of small and medium cranes, load-bearing parts of large-span workshops, connections of onshore wind power towers, and medium-pressure pipeline supports. Q690D is used for core load-bearing parts including coal mine hydraulic supports, where working resistance can rise from 8000 kN to 12000 kN, booms of 56-meter pump trucks and port cranes with weight savings of about 23 percent, and seismic supports of super high-rise buildings in northern regions. In cost terms, Q690D typically sells about 15 to 25 percent higher than Q620D because of stricter refining and alloy control. For general low-temperature and medium-load projects, Q620D is the economical choice; for heavy loads and harsh low temperatures, Q690D is preferred.
Frequently Asked Questions
What is the core difference between Q620D and Q690D? The core difference is yield strength: at least 620 MPa for Q620D against at least 690 MPa for Q690D for thicknesses up to 50 mm, with corresponding differences in tensile strength and toughness control.
What impact toughness do the two grades guarantee? Both guarantee at least 34 joules at minus 20 degrees Celsius, and some higher-standard Q690D products can achieve impact performance at minus 40 degrees Celsius.
What are the welding requirements for Q690D? Q690D requires low-hydrogen consumables, preheat of 150 to 200 degrees Celsius for thick plates, heat input controlled within 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 and medium crane frames, large-span workshop structures, wind power tower connections, and medium-pressure pipeline supports.
Where is Q690D commonly used? Q690D is used for coal mine hydraulic supports, pump truck booms, port cranes, and seismic supports and bridge components in cold northern regions.
How do the prices compare? Q690D typically costs about 15 to 25 percent more than Q620D because of stricter impurity control, refined microalloying, and higher processing demands.