Q690D is a kind of low-alloy high-strength structural steel widely used in the field of heavy-duty engineering and equipment manufacturing. It complies with Chinese national standards such as GB/T 16270 - 2019 and GB/T 1591 - 2018. It is especially favored in scenarios requiring medium and low temperature resistance and heavy load bearing, and its performance and application scenarios are highly targeted.
Grade Interpretation
- Q: Derived from the first letter of the Chinese Pinyin for "yield strength", which defines that the steel grade is classified according to the yield strength index.
- 690: Indicates that the minimum yield strength of the steel is 690 MPa when the thickness is ≤ 50mm. With the increase of the steel plate thickness, the yield strength value will decrease slightly, but it still maintains a high-strength level.
- D: Represents its quality grade. The core requirement is that the Charpy V-notch impact test must be qualified at -20℃. This grade is higher than Q690A, B, and C, and is specially designed to adapt to cold environment construction and equipment operation.
Core Performance Indicators
- Chemical composition: The composition is precisely controlled to balance strength and processability. The key components are strictly limited: carbon (C) ≤ 0.18% to ensure weldability; manganese (Mn) ≤ 2.0% to improve hardenability; harmful elements phosphorus (P) ≤ 0.030% and sulfur (S) ≤ 0.025% to avoid material embrittlement. In addition, microalloying elements such as niobium (Nb) and titanium (Ti) are added to refine the grain structure, and elements such as chromium (Cr) and nickel (Ni) are properly added to optimize toughness. Its carbon equivalent is generally not more than 0.55%, which lays a foundation for good welding performance.
- Mechanical properties: It has an excellent balance of strength and toughness. The tensile strength ranges from 770 MPa to 940 MPa, and the elongation is not less than 14%, which can prevent sudden brittle fracture when bearing dynamic loads. In the -20℃ impact test, the average impact energy of three samples is not less than 34 J (the actual measured value of high-quality batches can reach 50 - 80 J). Meanwhile, it has good cold bending performance. After 180° cold bending with a bending radius of 3 times the plate thickness, there is no crack, which is convenient for the forming processing of structural parts.
Key Production Processes and Delivery Status
- Main production processes: Most manufacturers adopt the quenching and tempering (QT) process for production. Some also use the thermo-mechanical control process for rolling. During production, the final rolling temperature is accurately controlled at about 810℃ ± 20℃ to avoid uneven grain size. Before delivery, strict inspections such as chemical composition analysis, tensile test, and low-temperature impact test are carried out. For large structural parts, non-destructive testing is also performed one by one to eliminate internal defects.
- Delivery status: The default delivery is in the quenched and tempered state. According to the needs of downstream processing, manufacturers can also provide customized processing such as cutting and bending. The common thickness of Q690D steel plates ranges from 6mm to 100mm, and the width and length can be customized, which is suitable for the production of various large structural parts.
Typical Application Scenarios
- Construction machinery: It is a key material for core components of heavy equipment such as Sany Heavy Industry and XCMG. It is used to make crane booms, pump truck outriggers, and excavator bucket rods. It can reduce the thickness of components by 30% - 50% while ensuring load-bearing capacity, realizing the lightweight of equipment and extending the service life by more than 20%.
- Bridge and building engineering: It has been applied in major projects such as the Hutong Yangtze River Bridge. It is used for chord members and truss structures of long-span bridges, as well as the main frame of high-rise buildings and seismic support components. It can reduce the amount of steel used while improving the anti-seismic and wind load resistance of the structure.
- Energy and mining equipment: In the energy field, it is used for wind power tower barrels and oil drilling platform pipe racks, which can resist the cyclic load and harsh outdoor environment. In the mining field, it is used to make hydraulic support columns and scraper conveyor middle troughs, which can withstand high pressure and impact loads and reduce the frequency of equipment maintenance.
- Ship and heavy-duty vehicles: It is used for frame and load-bearing parts of large oil tankers and bulk carriers, and can adapt to the corrosion of the marine environment and complex stress. It is also used to make frames and axles of heavy-duty trucks and large flatbed transport vehicles, improving the load-bearing capacity and durability of vehicles.
What specific performance requirement does the grade "D" in Q690D correspond to?
The letter "D" represents the quality grade of Q690D. Its core requirement is that the steel must pass the Charpy V-notch impact test at -20℃, with an average impact energy of no less than 47 J. Designed for cold environments, this grade offers better low-temperature toughness compared to Q690A, B and C grades. It can effectively prevent structural brittle fracture under low-temperature working conditions, making it suitable for engineering projects and equipment in frigid regions.
What key points should be noted when welding Q690D steel?
Although Q690D has good weldability, three core points need to be strictly controlled. First, welding material selection: prioritize low-hydrogen electrodes or 80%Ar + 20%CO₂ mixed gas shielded welding to reduce the risk of weld cold cracking. Second, temperature control: the preheating temperature should be adjusted according to plate thickness-100–150℃ for plates ≥12mm, and 150–200℃ for plates ≥25mm; the interpass temperature must not exceed 200℃ to avoid grain coarsening in the heat-affected zone. Third, post-weld treatment: for thick plates or critical structural components, perform hydrogen removal heat treatment at 550–600℃ after welding to eliminate residual stress and improve joint toughness.
How does Q690D differ from Q690E in terms of performance and application scenarios?
The core difference lies in low-temperature impact toughness requirements. Q690D meets the impact test standard at -20℃, while Q690E needs to pass the test at -40℃. In terms of chemical composition, Q690E has stricter control over harmful elements (phosphorus and sulfur) and adds more alloying elements to enhance ultra-low-temperature performance. For application scenarios, Q690D is suitable for cold regions with temperatures above -20℃, such as northern China's engineering machinery, wind turbine towers and bridge structures. Q690E is tailored for extreme cold environments like high-latitude areas and deep-sea projects, where temperatures drop below -30℃ for long periods.